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_view/contains.md

@@ -34,11 +34,11 @@ No-throw guarantee: this function never throws exceptions.
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
    another, in document order, stopping at the first match. Each comparison first checks the
-   key's length -- already known from the index, without reading the key bytes -- before comparing its content.
+   key's length — already known from the index, without reading the key bytes — before comparing its content.
    Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes
    constant time on average.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
-   that level or the index into the array -- as for [`operator[]`](operator[].md#complexity) and
+   that level or the index into the array — as for [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) with a JSON pointer.
 
 ## Notes
@@ -50,7 +50,7 @@ No-throw guarantee: this function never throws exceptions.
     `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
     the last value.
 
-Overload 1 always returns `#!cpp false` when the value is not an object -- including a [discarded](is_discarded.md)
+Overload 1 always returns `#!cpp false` when the value is not an object — including a [discarded](is_discarded.md)
 view.
 
 !!! info "Postconditions"
@@ -101,9 +101,9 @@ view.
 
 ## See also
 
-- [find](find.md) - find a value in an object
-- [count](count.md) - returns the number of occurrences of a key
-- [at](at.md), [operator[]](operator[].md) - resolve a JSON pointer and throw, or return a discarded view
+- [`find`](find.md) - find a value in an object
+- [`count`](count.md) - returns the number of occurrences of a key
+- [`at`](at.md), [`operator[]`](operator[].md) - resolve a JSON pointer and throw, or return a discarded view
 - [`BasicJsonType::contains`](../basic_json/contains.md) - the corresponding function of `basic_json`
 
 ## Version history