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/object_t.md

@@ -20,7 +20,7 @@ To store objects in C++, a type is defined by the template parameters described
 `ObjectType`
 :   the container to store objects. Its template parameters must have the same order and meaning as those of
     `std::map`; in particular, the third parameter is a comparator. `#!cpp std::unordered_map`, whose third parameter
-    is a hash function, therefore needs an adapter -- see
+    is a hash function, therefore needs an adapter — see
     [Template Parameter Requirements](../../features/types/template_parameters.md#objecttype) for the full list of
     requirements, an adapter example, and the containers that are known to work.
 
@@ -125,10 +125,12 @@ the object is silently converted as an array of key-value pairs, which is incorr
 
 ## See also
 
-- [array_t](array_t.md) the type used to store JSON arrays
-- [string_t](string_t.md) the type used to store JSON strings
-- [object_comparator_t](object_comparator_t.md) the comparator used to order object keys
+- [`array_t`](array_t.md) - the type used to store JSON arrays
+- [`string_t`](string_t.md) - the type used to store JSON strings
+- [`object_comparator_t`](object_comparator_t.md) - the comparator used to order object keys
 - [Object Order](../../features/object_order.md) - the article on object key ordering
+- [Types](../../features/types/index.md) - how objects are stored
+- [Template Parameter Requirements](../../features/types/template_parameters.md) - requirements on the `ObjectType` template parameter
 
 ## Version history