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/basic_json.md

@@ -52,7 +52,7 @@ basic_json(basic_json&& other) noexcept;
     The postcondition of this constructor can be restored by calling [`clear()`](clear.md).
 
 2. Create a `#!json null` JSON value. It either takes a null pointer as parameter (explicitly creating `#!json null`)
-   or no parameter (implicitly creating `#!json null`). The passed null pointer itself is not read -- it is only used to
+   or no parameter (implicitly creating `#!json null`). The passed null pointer itself is not read — it is only used to
    choose the right constructor.
 
 3. This is a "catch all" constructor for all compatible JSON types; that is, types for which a `to_json()` method
@@ -113,19 +113,22 @@ basic_json(basic_json&& other) noexcept;
 
     ```mermaid
     flowchart TD
-        A(["initializer_list init"]) --> B{"empty, or every element is a 2-element<br/>array whose first element is a string?"}
-        B -->|"yes"| C{"type_deduction"}
-        B -->|"no"| D{"type_deduction"}
+        A(["initializer_list init"]) --> B{{"empty, or every element is a 2-element<br/>array whose first element is a string?"}}
+        B -->|"yes"| C{{"type_deduction"}}
+        B -->|"no"| D{{"type_deduction"}}
         C -->|"true"| OBJ["create object"]
-        C -->|"false"| E{"manual_type"}
+        C -->|"false"| E{{"manual_type"}}
         E -->|"object"| OBJ
         E -->|"array"| ARR["create array"]
         D -->|"true"| ARR
-        D -->|"false"| F{"manual_type"}
+        D -->|"false"| F{{"manual_type"}}
         F -->|"array"| ARR
         F -->|"object"| ERR["throw type_error.301"]
     ```
 
+    <p class="nj-caption" markdown>Decision flow for an initializer list: hexagons are checks, edge labels are their
+    outcomes, and rectangles are the resulting value or exception.</p>
+
     The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows:
     
     1. The empty initializer list is written as `#!cpp {}` which is exactly an empty JSON object.
@@ -473,12 +476,13 @@ basic_json(basic_json&& other) noexcept;
 
 ## See also
 
-- [array](array.md) create a JSON array value, forcing array creation from an initializer list even when it looks like
+- [`array`](array.md) - create a JSON array value, forcing array creation from an initializer list even when it looks like
   an object
-- [object](object.md) create a JSON object value, forcing object creation from an initializer list
-- [binary](binary.md) create a JSON binary array value
-- [operator=](operator=.md) copy assignment operator
+- [`object`](object.md) - create a JSON object value, forcing object creation from an initializer list
+- [`binary`](binary.md) - create a JSON binary array value
+- [`operator=`](operator=.md) - copy assignment operator
 - [Creating JSON values](../../features/creating_values.md) - the article on creating JSON values
+- [Converting values](../../features/conversions.md) - which C++ types can be converted
 
 ## Version history