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_view/value.md

@@ -12,8 +12,8 @@ T value(const json_pointer& ptr, const T& default_value) const;
 string_t value(const json_pointer& ptr, const char* default_value) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
-   [Notes on duplicate keys](operator[].md#notes)) -- converted to `T`, or `default_value` if there is no such member.
+1. Returns the value of the object member with key `key` — the first one, should the key occur more than once (see
+   [Notes on duplicate keys](operator[].md#notes)) — converted to `T`, or `default_value` if there is no such member.
 2. Returns the value a JSON pointer `ptr` refers to, starting at this value, converted to `T`, or `default_value` if
    `ptr` cannot be resolved.
 
@@ -48,16 +48,16 @@ Strong guarantee: there are no changes to the view or the document it refers to.
 
 ## Exceptions
 
-1. Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if the value is not an object --
+1. Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if the value is not an object —
    the same exception, with the same message, that [`BasicJsonType::value`](../basic_json/value.md) throws for the
    same call. If a member with key `key` is found, throws whatever converting it to `T` throws (typically
    [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302), with the same message
    [`BasicJsonType::value`](../basic_json/value.md) throws for the same mismatch); a missing member never throws.
-2. Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if this value -- not the value `ptr`
-   resolves to -- is neither an object nor an array. Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106)
+2. Throws [`type_error.306`](../../home/exceptions.md#jsonexceptiontype_error306) if this value — not the value `ptr`
+   resolves to — is neither an object nor an array. Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106)
    or [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if `ptr` contains a malformed array
    index. If `ptr` resolves to a value, throws whatever converting it to `T` throws. Every other way `ptr` can fail to
-   resolve -- a missing key, an out-of-range or "`-`" array index, an unresolvable token on a primitive -- yields
+   resolve — a missing key, an out-of-range or "`-`" array index, an unresolvable token on a primitive — yields
    `default_value` instead of throwing, exactly as [`BasicJsonType::value`](../basic_json/value.md) catches
    `out_of_range` and returns `default_value`.
 
@@ -73,7 +73,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
    Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
    constant time on average.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
-   that level or the index into the array -- as for the [`operator[]`](operator[].md#complexity) and
+   that level or the index into the array — as for the [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) overloads that take a JSON pointer. Plus the complexity of converting the resolved value
    to `T`.
 
@@ -89,7 +89,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 !!! info "Differences to `at` and `operator[]`"
 
     Unlike [`at`](at.md), this function does not throw if `key`/`ptr` resolves to no value. Unlike
-    [`operator[]`](operator[].md), it never returns a [discarded](is_discarded.md) view -- it always returns a `T` --
+    [`operator[]`](operator[].md), it never returns a [discarded](is_discarded.md) view — it always returns a `T` —
     and it is available on any view, since it never needs to insert a missing element the way the non-const
     `BasicJsonType::operator[]` would.
 
@@ -131,8 +131,8 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 
 ## See also
 
-- [at](at.md) - access specified element with bounds checking (throws instead of returning a default value)
-- [operator[]](operator[].md) - access specified element (returns a discarded view instead of a default value)
+- [`at`](at.md) - access specified element with bounds checking (throws instead of returning a default value)
+- [`operator[]`](operator[].md) - access specified element (returns a discarded view instead of a default value)
 - [`BasicJsonType::value`](../basic_json/value.md) - the corresponding function of `basic_json`
 
 ## Version history