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/get.md

@@ -7,26 +7,26 @@ T get() const;
 
 Converts the value to `T`.
 
-For the types below, the conversion works directly on the flat index -- no `BasicJsonType` value is built for it:
+For the types below, the conversion works directly on the flat index — no `BasicJsonType` value is built for it:
 
 - `#!cpp bool`
 - arithmetic types other than `#!cpp bool` (for a boolean, `#!cpp 0`/`#!cpp 1`, exactly as for
   [`BasicJsonType::get<T>()`](../basic_json/get.md))
 - `#!cpp std::nullptr_t`
-- `#!cpp std::basic_string<char, Traits, Alloc>` (including `string_t`) -- a copy of the string
-- [`string_view_t`](index.md#member-types) -- **no copy**: the returned view points into the document's
+- `#!cpp std::basic_string<char, Traits, Alloc>` (including `string_t`) — a copy of the string
+- [`string_view_t`](index.md#member-types) — **no copy**: the returned view points into the document's
   [`source()`](../basic_json_document/source.md) text, or, for a string that contains escape sequences, into the
   document's own buffer of decoded strings (see [`get_string()`](get_string.md))
-- `BasicJsonType` -- equivalent to [`materialize()`](materialize.md)
-- `basic_json_view` -- returns `#!cpp *this`
-- `#!cpp std::vector<U, A>` of the types above -- from an array, element by element, each converted with
+- `BasicJsonType` — equivalent to [`materialize()`](materialize.md)
+- `basic_json_view` — returns `#!cpp *this`
+- `#!cpp std::vector<U, A>` of the types above — from an array, element by element, each converted with
   `#!cpp get<U>()`; `#!cpp std::vector<basic_json_view>` keeps a view of every element instead of a value
 - `#!cpp std::map<K, basic_json_view, C, A>` and `#!cpp std::unordered_map<K, basic_json_view, H, E, A>`, if `K` is
-  constructible from a `#!cpp (const char*, std::size_t)` pair -- views of the members of an object; with a repeated
+  constructible from a `#!cpp (const char*, std::size_t)` pair — views of the members of an object; with a repeated
   key, the view of the *last* member is kept, as [`BasicJsonType::parse()`](../basic_json/parse.md) keeps its value
 
 A default-constructible `T` whose `from_json()` accepts views reads the view directly as well, without building a
-`BasicJsonType` value. This covers the library's own conversions of the standard library types -- all other
+`BasicJsonType` value. This covers the library's own conversions of the standard library types — all other
 `#!cpp std::vector`s, `#!cpp std::map`/`#!cpp std::unordered_map` (also with other key types), `#!cpp std::list`,
 `#!cpp std::deque`, `#!cpp std::set`, `#!cpp std::forward_list`, `#!cpp std::valarray`, `#!cpp std::array`, C arrays,
 `#!cpp std::pair`, `#!cpp std::tuple`, `#!cpp std::optional`, enumerations, `#!cpp std::filesystem::path`, ... --
@@ -47,8 +47,8 @@ T&)` of [arbitrary type conversions](../../features/arbitrary_types.md), is not
     keeps it: for `#!cpp {"x":1,"x":2}`, `#!cpp get<point>()` reads `#!cpp x == 1`. See
     [Duplicate keys](../../features/json_view.md#duplicate-keys).
 
-Every other `T` -- user types whose `from_json()` is written for `BasicJsonType` only, types without a default
-constructor, `#!cpp std::tuple`s holding references, ... -- is converted by `#!cpp materialize().get<T>()`: the subtree is built into a real `BasicJsonType`
+Every other `T` — user types whose `from_json()` is written for `BasicJsonType` only, types without a default
+constructor, `#!cpp std::tuple`s holding references, ... — is converted by `#!cpp materialize().get<T>()`: the subtree is built into a real `BasicJsonType`
 value first (as [`BasicJsonType::parse()`](../basic_json/parse.md) would), and converted from there exactly as
 [`BasicJsonType::get<T>()`](../basic_json/get.md) would convert it.
 
@@ -69,7 +69,7 @@ Strong guarantee: there are no changes to the view or the document it refers to.
 
 - For the directly-converted types listed above (other than `BasicJsonType` and `basic_json_view`, which never
   throw): throws [`type_error.302`](../../home/exceptions.md#jsonexceptiontype_error302) if the value's type does not
-  match `T` -- the same exception, with the same message, that [`BasicJsonType::get<T>()`](../basic_json/get.md)
+  match `T` — the same exception, with the same message, that [`BasicJsonType::get<T>()`](../basic_json/get.md)
   throws for the same JSON type and `T`.
 - For `#!cpp std::vector<U, A>`: throws `type_error.302` if the value is not an array; otherwise, whatever converting
   an element to `U` throws.
@@ -77,7 +77,7 @@ Strong guarantee: there are no changes to the view or the document it refers to.
 - For a `T` whose `from_json()` reads the view: whatever that `from_json()` throws; for the library's conversions and
   the macros, the same exceptions with the same messages as [`BasicJsonType::get<T>()`](../basic_json/get.md) of the
   same value.
-- For every other `T`: whatever [`materialize().get<T>()`](../basic_json/get.md) throws -- typically `type_error.302`,
+- For every other `T`: whatever [`materialize().get<T>()`](../basic_json/get.md) throws — typically `type_error.302`,
   or whatever a user-provided `from_json()` throws.
 
 None of the exceptions thrown directly by this function or by a `from_json()` that reads the view (the first four
@@ -132,7 +132,7 @@ document.
 
     The example below reads typed fields straight into C++ variables, collects a view of every array element with
     `#!cpp get<std::vector<basic_json_view>>()` instead of a value, and converts a nested object into a user type
-    through its `from_json()` -- which runs on a `BasicJsonType` value `materialize()` builds for just that one
+    through its `from_json()` — which runs on a `BasicJsonType` value `materialize()` builds for just that one
     member.
 
     ```cpp
@@ -147,11 +147,12 @@ document.
 
 ## See also
 
-- [get_to](get_to.md) - convert and write into a passed value
-- [get_string](get_string.md) - the string, without a copy
-- [number_token](number_token.md) - a number's token text, without a copy
-- [materialize](materialize.md) - build the `BasicJsonType` value of this subtree
+- [`get_to`](get_to.md) - convert and write into a passed value
+- [`get_string`](get_string.md) - the string, without a copy
+- [`number_token`](number_token.md) - a number's token text, without a copy
+- [`materialize`](materialize.md) - build the `BasicJsonType` value of this subtree
 - [`BasicJsonType::get`](../basic_json/get.md) - the corresponding function of `basic_json`
+- [Zero-copy JSON views](../../features/json_view.md#getting-values-out-without-copying) - which conversions avoid building a basic_json value
 
 ## Version history