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/index.md

@@ -189,7 +189,7 @@ The class satisfies the following concept requirements:
 Functions to inspect the type of a JSON value.
 
 - [**type**](type.md) - return the type of the JSON value
-- [**operator value_t**](operator_value_t.md) - return the type of the JSON value
+- [`operator value_t`](operator_value_t.md) - return the type of the JSON value
 - [**type_name**](type_name.md) - return the type as string
 - [**is_primitive**](is_primitive.md) - return whether the type is primitive
 - [**is_structured**](is_structured.md) - return whether the type is structured
@@ -218,7 +218,7 @@ Direct access to the stored value of a JSON value.
 - [**get_to**](get_to.md) - get a value and write it to a destination
 - [**get_ptr**](get_ptr.md) - get a pointer value
 - [**get_ref**](get_ref.md) - get a reference value
-- [**operator ValueType**](operator_ValueType.md) - get a value
+- [`operator ValueType`](operator_ValueType.md) - get a value
 - [**get_binary**](get_binary.md) - get a binary value
 - [**as_base_class**](as_base_class.md) - access the custom base class
 
@@ -359,10 +359,15 @@ Access to the JSON value
 
 ## See also
 
-- [RFC 8259: The JavaScript Object Notation (JSON) Data Interchange Format](https://tools.ietf.org/html/rfc8259)
-- [basic_json_document](../basic_json_document/index.md) / [basic_json_view](../basic_json_view/index.md) - parse a JSON
+- [RFC 8259: The JavaScript Object Notation (JSON) Data Interchange Format](https://tools.ietf.org/html/rfc8259) -
+  specification of the JSON data interchange format
+- [`basic_json_document`](../basic_json_document/index.md) - / [`basic_json_view`](../basic_json_view/index.md) - parse a JSON
   text into a flat index and read it through a view
 - [Zero-copy JSON views](../../features/json_view.md) - when to use a document and a view instead of a `basic_json`
+- [Types](../../features/types/index.md) - how JSON values are stored and which types exist
+- [Converting values](../../features/conversions.md) - get values out of a JSON value and put values in
+- [Creating JSON values](../../features/creating_values.md) - all the ways to build a JSON value
+- [Arbitrary Type Conversions](../../features/arbitrary_types.md) - convert your own types to and from JSON
 
 ## Version history