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/namespace.md

@@ -38,6 +38,22 @@ For example, the namespace name for version 3.11.2 with `JSON_DIAGNOSTICS` defin
 nlohmann::json_abi_diag_v3_11_2
 ```
 
+The name is assembled from these segments; pick a version and macros to see the resulting name:
+
+<figure class="nj-integration-ns" data-nj-integration-ns aria-label="How the inline namespace name is assembled from its segments">
+<div class="nj-integration-ns__strip" aria-live="polite">
+<span class="nj-integration-ns__seg nj-integration-ns__seg--root"><code>nlohmann</code><small>root namespace</small></span>
+<span class="nj-integration-ns__seg nj-integration-ns__seg--sep"><code>::</code><small>&nbsp;</small></span>
+<span class="nj-integration-ns__seg nj-integration-ns__seg--abi"><code>json_abi</code><small>always</small></span>
+<span class="nj-integration-ns__seg nj-integration-ns__seg--tag"><code>_diag</code><small>JSON_DIAGNOSTICS=1</small></span>
+<span class="nj-integration-ns__seg nj-integration-ns__seg--ver"><code>_v3_11_2</code><small>version 3.11.2</small></span>
+</div>
+<figcaption>Version 3.11.2 with <code>JSON_DIAGNOSTICS</code> defined to <code>1</code>. ABI tags are appended in the order of the
+list above, the version suffix comes last.</figcaption>
+</figure>
+
+<p class="nj-caption" markdown>The name of the inline namespace, assembled from its segments. Select a version and macros to see which ABI tags appear; a tag the chosen version does not have is disabled.</p>
+
 ## Purpose
 
 Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different
@@ -55,6 +71,8 @@ graph
     app --> library
 ```
 
+<p class="nj-caption" markdown>The application uses the library with `JSON_DIAGNOSTICS=1` and links `some library`, which uses the same library version with `JSON_DIAGNOSTICS=0`. Arrows point from a component to what it uses.</p>
+
 In releases prior to 3.11.0, mixing any version of the JSON library with different `JSON_DIAGNOSTICS` settings would
 result in a crashing application. If `some_library` never passes instances of JSON library types to the application,
 this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.
@@ -102,6 +120,15 @@ follows:
     Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You
     have been warned!
 
+## See also
+
+- [`NLOHMANN_JSON_NAMESPACE`](../api/macros/nlohmann_json_namespace.md) - the library namespace as macro
+- [`NLOHMANN_JSON_NAMESPACE_BEGIN`, `NLOHMANN_JSON_NAMESPACE_END`](../api/macros/nlohmann_json_namespace_begin.md) - open and close the library namespace
+- [`NLOHMANN_JSON_NAMESPACE_NO_VERSION`](../api/macros/nlohmann_json_namespace_no_version.md) - disable the version component
+- [Supported Macros](macros.md) - all supported macros
+- [Migration Guide](../integration/migration_guide.md) - do not hard-code the library namespace
+- [`basic_json`](../api/basic_json/index.md) - the main class and all its member functions
+
 ## Version history
 
 - Introduced inline namespace (`json_v3_11_0[_abi-tag]*`) in version 3.11.0.