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/push_back.md

@@ -8,8 +8,8 @@ view_type push_back(view_type array, V&& value);
 Appends `value` as a new last element of `array`. A [null](../basic_json_view/is_null.md) `array` first becomes an
 empty array, the same way [`set`](set.md) turns a null `object` into an empty object.
 
-`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 `array`'s own document -- which is copied so that nothing is shared with the
+`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 `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, ...).
 
@@ -35,23 +35,23 @@ a view of the new last element of `array`, now holding `value`
 
 ## 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 `array` switches from its parsed
-layout to a growable block, or while that block grows, see [Notes](#notes) -- can still leave a partial effect, such
+for the encoding that is not reclaimed. A failure of a later allocation — while `array` switches from its parsed
+layout to a growable block, or while that block grows, see [Notes](#notes) — can still leave a partial effect, such
 as a null `array` argument already turned into an empty array even though `value` itself was not appended.
 
 ## Exceptions
 
 Throws [`type_error.308`](../../home/exceptions.md#jsonexceptiontype_error308) if `array` is neither an array nor
-null -- the same message [`BasicJsonType::push_back`](../basic_json/push_back.md) throws for the same type. Throws
+null — the same message [`BasicJsonType::push_back`](../basic_json/push_back.md) throws for the same type. Throws
 [`invalid_iterator.202`](../../home/exceptions.md#jsonexceptioninvalid_iterator202) ("view does not belong to this
 document") if `array` is a [discarded](../basic_json_view/is_discarded.md) view or a view of a *different* document.
 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, and [`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. Throws
+binary value — `BasicJsonType` can hold one, but a `json_document` cannot. Throws
 [`type_error.316`](../../home/exceptions.md#jsonexceptiontype_error316) if `value` is (or contains) a string that is
 not valid UTF-8, with the same message [`BasicJsonType::dump()`](../basic_json/dump.md) gives for that string.
 
@@ -59,14 +59,14 @@ not valid UTF-8, with the same message [`BasicJsonType::dump()`](../basic_json/d
 
 Amortized constant, plus time linear in the size of `value` to encode it into the document's storage (constant for
 a scalar, linear in the number of nested values for an array or object): the elements of `array` move to a growable
-block of links the first time it is appended to (or [`set`](set.md) on), and that block itself grows -- doubling its
-capacity, so the cost of growing it amortizes to constant per element -- only once it runs out of room. See
+block of links the first time it is appended to (or [`set`](set.md) on), and that block itself grows — doubling its
+capacity, so the cost of growing it amortizes to constant per element — only once it runs out of room. See
 [Notes](#notes).
 
 ## Notes
 
-Like [`set`](set.md) on a member or an element, `push_back` never moves an existing element itself -- only where
-`array`'s *links* to its elements live -- so a view of an existing element of `array` stays valid across a
+Like [`set`](set.md) on a member or an element, `push_back` never moves an existing element itself — only where
+`array`'s *links* to its elements live — so a view of an existing element of `array` stays valid across a
 `push_back`, but any iterator already taken over `array` is invalidated, since it was walking the old layout. See
 [Edits](index.md#edits) for what stays valid across an edit in general.
 
@@ -90,12 +90,13 @@ Like [`set`](set.md) on a member or an element, `push_back` never moves an exist
 
 ## See also
 
-- [set](set.md) - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
-- [insert](insert.md) - insert an element into an array before a given position
-- [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
+- [`set`](set.md) - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
+- [`insert`](insert.md) - insert an element into an array before a given position
+- [`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
 - [`BasicJsonType::push_back`](../basic_json/push_back.md) - the corresponding function of `basic_json`
 - [Edits](index.md#edits) - what an edit guarantees, for every overload
+- [Zero-copy JSON views](../../features/json_view.md#editing-a-document) - what an edit guarantees and costs
 
 ## Version history