docs: new design for the documentation site (#5934)

* docs: new design for the documentation site

Apply the brand to the Material for MkDocs site: Inter and JetBrains Mono
served by the site (upright and real italic), brand colours, blue header band
with the syntax pattern, wordmark in the header, new home page (hero, cards,
animated feature tour), 404 page, polish for tabs, search, footer, focus
rings, tables and code blocks, and theme features (code copy, previous/next
footer, TOC follow, search suggestions).

Draw the number railroad diagram, the parser callback events and the iterator
illustrations as inline SVGs that follow the light and dark scheme. The
generator scripts are in docs/mkdocs/scripts/figures; the feature slideshow
(json-light.svg, json-dark.svg, replacing json.gif on the home page) is built
by docs/mkdocs/scripts/slideshow. Decision nodes in five flowcharts become
hexagons, which need far less room.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: add figures, table captions and a macro chooser

- Figures with numbered captions: SAX stepper, json_view layout and edit
  stepper, exception map, JSON pointer and patch walkers, QA pipeline,
  scaling and binary size charts, roadmap lanes, release timeline,
  integration decision tree and namespace builder
- Table captions (nj-table-caption) on 9 pages
- Compile times: modules and PCH charts, filesystem section, re-measured
  Figure 1, links between features/modules.md and compile_times.md
- Interactive macro chooser for features/arbitrary_types.md
- Link check: exclude URLs that answer the plugin with 400/401/419 or are
  dead (customers.md)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: macro chooser generates adl_serializer for non-default-constructible types

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: cross-link concept pages and API pages in See also sections

Add See also links in both directions between the concept pages (features,
home, integration) and the API pages, write API links in code font, give every
entry a comment after a dash, and show the callback event figure on the parser
callbacks page.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* docs: address review comments on the new design

- Home: add a "Latest release" button
- Sponsors: update the list and group it by tier
- Features: list all feature pages on the index page
- Releases: add highlights per release, link the 4.0 roadmap, add the first commit to the timeline
- Exceptions: prefix all example messages with the exception name
- API: no code font for format names in the See also links of the to_/from_ pages
- Merge Patch: show the document before, the patch, and the result
- Replace "--" with typographic dashes, mark external links with an icon, center Mermaid diagrams

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_document/set.md

@@ -23,8 +23,8 @@ has `set`; calling it on a read-only `basic_json_document` fails to compile (`#!
 
 1. Replaces the value `target` refers to with `value`.
 2. Sets the member `key` of the object `object` to `value`: assigns it if `object` already has a member with this
-   key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the
-   [Notes](#notes) below) -- or appends a new member at the end otherwise. A [null](../basic_json_view/is_null.md)
+   key — the first one, should the key occur more than once, and the later duplicates are then dropped (see the
+   [Notes](#notes) below) — or appends a new member at the end otherwise. A [null](../basic_json_view/is_null.md)
    `object` first becomes an empty object.
 3. Assigns `value` to the element at index `idx` of the array `array`, which must already exist (`#!cpp idx <
    array.size()`).
@@ -37,7 +37,7 @@ has `set`; calling it on a read-only `basic_json_document` fails to compile (`#!
    null values up to that index), an object otherwise. An empty `ptr` sets [`root()`](root.md) itself, as in 1.
 
 In every overload, `value` is accepted three ways: a [`basic_json_view`](../basic_json_view/index.md) of *any*
-document -- read-only or editable, and it does not have to be `target`'s/`object`'s/`array`'s own document -- which
+document — read-only or editable, and it does not have to be `target`'s/`object`'s/`array`'s own document — which
 is copied so that nothing is shared with the source document afterward; a `BasicJsonType` value; or anything
 `BasicJsonType` can be constructed from (numbers, strings, `#!cpp bool`, `#!cpp nullptr`, containers, ...).
 
@@ -82,11 +82,11 @@ is copied so that nothing is shared with the source document afterward; a `Basic
 
 ## Exception safety
 
-Basic exception safety: `value` is fully encoded -- including the checks below -- into storage owned by the document
+Basic exception safety: `value` is fully encoded — including the checks below — into storage owned by the document
 before anything already reachable from [`root()`](root.md) is touched, so a failure while encoding `value` (an
 invalid argument, or `#!cpp std::bad_alloc`) leaves the document completely unchanged, other than memory allocated
-for the encoding that is not reclaimed. A failure of a later allocation -- while an edited array or object switches
-from its parsed layout to a growable block, see [Notes](#notes) -- can still leave a partial effect, such as a
+for the encoding that is not reclaimed. A failure of a later allocation — while an edited array or object switches
+from its parsed layout to a growable block, see [Notes](#notes) — can still leave a partial effect, such as a
 [null](../basic_json_view/is_null.md) `object`/`array` argument already turned into an empty object/array even
 though `value` itself was not linked in. Likewise, a failure of `value` in 4. leaves a null parent that is set with an
 index beyond 0 already filled with the null values before the index.
@@ -95,14 +95,14 @@ index beyond 0 already filled with the null values before the index.
 
 1. Throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if `value` is a
    [discarded](../basic_json_view/is_discarded.md) view, or a [discarded](../basic_json/is_discarded.md)
-   `BasicJsonType` value (e.g. `#!cpp BasicJsonType(value_t::discarded)`) -- an object or array `value`, of either
+   `BasicJsonType` value (e.g. `#!cpp BasicJsonType(value_t::discarded)`) — an object or array `value`, of either
    kind, is fine and is encoded as a whole subtree.
 2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if `object` is neither an object
-   nor null -- the same message [`operator[]`](../basic_json_view/operator%5B%5D.md) throws for a string argument on
+   nor null — the same message [`operator[]`](../basic_json_view/operator%5B%5D.md) throws for a string argument on
    such a value. Throws [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) if `key` is not
    valid UTF-8, with the same message [`BasicJsonType::dump()`](../basic_json/dump.md) gives for that string.
    Also throws what 1. throws for `value`.
-3. Throws `type_error.305` if `array` is not an array -- the same message `operator[]` throws for a numeric argument
+3. Throws `type_error.305` if `array` is not an array — the same message `operator[]` throws for a numeric argument
    on such a value. Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if `idx` is
    negative, or if `#!cpp idx >= array.size()`. Also throws what 1. throws for `value`.
 4. Throws what [`at`](../basic_json_view/at.md) throws (overload 3) for resolving `ptr`'s parent, except that a
@@ -121,7 +121,7 @@ index beyond 0 already filled with the null values before the index.
    Also throws what 1. throws for `value`.
 
 Every overload also throws [`type_error.319`](../../home/exceptions.md#jsonexceptiontype_error319) if `value` is (or
-contains) a binary value -- `BasicJsonType` can hold one, but a `json_document` cannot -- and
+contains) a binary value — `BasicJsonType` can hold one, but a `json_document` cannot — and
 [`invalid_iterator.202`](../../home/exceptions.md#jsonexceptioninvalid_iterator202) ("view does not belong to this
 document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_discarded.md) view or a view of a
 *different* document (overloads 1-3 only; overload 4 always starts from this document's own [`root()`](root.md)).
@@ -132,7 +132,7 @@ document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_d
    number of nested values for an array or object. If `target` is itself an array or object that spans more than one
    node in its parent's original, unedited layout, and `value` is a scalar, replacing it additionally costs time
    linear in the size of the document, the *first* time (the parent of `target` is looked up from
-   [`root()`](root.md), and then switches to links) -- see [Notes](#notes). Once the parent has links, `target` is
+   [`root()`](root.md), and then switches to links) — see [Notes](#notes). Once the parent has links, `target` is
    replaced in constant time: setting every element of a large array one after the other is linear overall. To avoid
    the lookup altogether, use 3. (or 2. for an object), which know the parent.
 2. Linear in the number of members of `object`, to find an existing member with `key`, plus the complexity of 1. for
@@ -147,7 +147,7 @@ document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_d
 !!! info "Duplicate keys"
 
     If `object` already has more than one member with `key` (2.), the *first* one is assigned `value` and every
-    later member with the same key is removed -- so that a lookup, an iteration, and
+    later member with the same key is removed — so that a lookup, an iteration, and
     [`materialize()`](../basic_json_view/materialize.md) of `object` afterward all agree on a single value for
     `key`, the same way [`operator[]`](../basic_json_view/operator%5B%5D.md) already picks the first occurrence of a
     duplicate key for reading. See the [Notes on duplicate keys](../basic_json_view/operator%5B%5D.md#notes) of
@@ -156,8 +156,8 @@ document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_d
 Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited
 before switches it from its parsed layout to a growable block holding links to its elements; a later
 [`push_back`](push_back.md) or `set` on the same container reuses that block, growing it (amortized constant time)
-only once it runs out of room. This never moves an element itself -- only where the container's *links* to its
-elements live -- so a view of an element stays valid, but any iterator already taken over the container is
+only once it runs out of room. This never moves an element itself — only where the container's *links* to its
+elements live — so a view of an element stays valid, but any iterator already taken over the container is
 invalidated, since it was walking the old layout. See [Edits](index.md#edits) for what stays valid across an edit in
 general.
 
@@ -170,8 +170,8 @@ one-node scalar.
 
 ??? example "Example: (1)/(2)/(3)/(4) replace a value, set a member, assign an element, set via a JSON pointer"
 
-    The example below edits a small configuration document -- replacing a value, adding an object member, assigning
-    an array element, and reaching a field through a JSON pointer -- and shows what
+    The example below edits a small configuration document — replacing a value, adding an object member, assigning
+    an array element, and reaching a field through a JSON pointer — and shows what
     [`dump()`](../basic_json_view/dump.md) preserves that is lost once the same edits are made on a `BasicJsonType`
     value instead: the order object members were written in, and the exact spelling of a number that was never
     touched.
@@ -188,10 +188,10 @@ one-node scalar.
 
 ## See also
 
-- [push_back](push_back.md) - append to an array
-- [insert](insert.md) - insert an element into an array
-- [erase](erase.md) - remove an object member, an array element, or the value a JSON pointer refers to
-- [root](root.md) - the view of the root value, the starting point of overload 4
+- [`push_back`](push_back.md) - append to an array
+- [`insert`](insert.md) - insert an element into an array
+- [`erase`](erase.md) - remove an object member, an array element, or the value a JSON pointer refers to
+- [`root`](root.md) - the view of the root value, the starting point of overload 4
 - [`basic_json_view::dump`](../basic_json_view/dump.md) - serialize the document, keeping an untouched number's
   spelling with `#!cpp number_format::source`
 - [Edits](index.md#edits) - what an edit guarantees, for every overload