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/features/parsing/parser_callbacks.md

@@ -31,6 +31,13 @@ table describes the values of the parameters `depth`, `event`, and `parsed`.
 | `parse_event_t::array_end`    | the parser read `]` and finished processing a JSON array  | depth of the parent of the JSON array     | the parsed JSON array            |
 | `parse_event_t::value`        | the parser finished reading a JSON value                  | depth of the value                        | the parsed JSON value            |
 
+<div class="nj-fig-wrap" markdown>
+--8<-- "images/callback_events.svg"
+</div>
+
+<p class="nj-caption" markdown>Each coloured dot marks where the parser reports a `parse_event_t` event while reading
+the JSON text on the left; the legend on the right names the event per colour.</p>
+
 ??? example "Example: sequence of callback events"
 
     When parsing the following JSON text,
@@ -95,7 +102,7 @@ was called:
 
 The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in
 [`object_t`](../../api/basic_json/object_t.md#behavior), it is unspecified which value for a repeated key ends up in
-the resulting `#!c json` value -- once parsing has produced that value, the duplicate is already gone, because object
+the resulting `#!c json` value — once parsing has produced that value, the duplicate is already gone, because object
 storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback
 can detect them while the object is still being read, before that ambiguity ever applies.
 
@@ -122,7 +129,7 @@ This approach has three limitations:
   maintaining a stack of the enclosing keys and array indices in the callback as well.
 
 A [SAX interface](sax_interface.md) does not lift the position limitation: its `key` function receives no position
-either -- only `parse_error` is passed the byte position.
+either — only `parse_error` is passed the byte position.
 
 ## Recipe: streaming a large homogeneous array
 
@@ -171,3 +178,11 @@ tracking the maximum `depth` seen and throwing once it is exceeded:
 
     json::parse(input, callback);
     ```
+
+## See also
+
+- [Parsing](index.md) - overview of the parsing functions
+- [SAX Interface](sax_interface.md) - guide to event-based parsing
+- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
+- [`parser_callback_t`](../../api/basic_json/parser_callback_t.md) - callback function type for the parser
+- [`parse_event_t`](../../api/basic_json/parse_event_t.md) - enumeration of parser events