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/index.md

@@ -13,6 +13,8 @@ flowchart LR
     A --> V["bool: is the input valid JSON?"]
 ```
 
+<p class="nj-caption" markdown>The three entry points for parsing: `parse()` builds a `basic_json` value, optionally filtered by a parser callback, `sax_parse()` delivers events to a SAX handler, and `accept()` only checks whether the input is valid.</p>
+
 ## Input
 
 The [`parse`](../../api/basic_json/parse.md) function reads a JSON value from an input. The input can be
@@ -34,7 +36,7 @@ json data = json::parse(f);
 ```
 
 The input must be encoded in UTF-8; other encodings are not supported. The exception is wide-character input, which
-is read as UTF-16 or UTF-32 -- depending on the character type -- and converted to UTF-8; see the
+is read as UTF-16 or UTF-32 — depending on the character type — and converted to UTF-8; see the
 [`parse`](../../api/basic_json/parse.md) page for an example. A single input may contain only one JSON value.
 Inputs consisting of multiple values separated by newlines are handled by the [JSON Lines](json_lines.md) format; input
 that arrives in chunks or contains several values is covered in
@@ -154,4 +156,11 @@ options.
 - [Zero-copy JSON views](../json_view.md) - parse into a read-only document and read it through views
 - [parsing and exceptions](parse_exceptions.md) - control error handling
 - [parsing untrusted input](untrusted_input.md) - what to consider when parsing input from untrusted sources
-- [Zero-copy JSON views](../json_view.md) - parse into a document and read it through a view
+- [`operator>>`](../../api/operator_gtgt.md) - deserialize from stream
+- [`json_sax`](../../api/json_sax/index.md) - documentation of the SAX interface
+- [`parse_error`](../../api/basic_json/parse_error.md) - exception thrown on a parse error
+- [Comments](../comments.md) - ignore `//` and `/* */` comments while parsing
+- [Trailing Commas](../trailing_commas.md) - accept trailing commas in arrays and objects
+- [Serialization](../serialization.md) - pretty-printing, UTF-8 handling, and number output
+- [Working Without Exceptions](../no_exceptions.md) - error handling when exceptions are disabled
+- [Exceptions](../../home/exceptions.md) - list of exceptions and error codes