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_document/parse.md

@@ -24,7 +24,7 @@ Both overloads accept the same JSON text as [`BasicJsonType::parse()`](../basic_
 the input) instead of a tree of `BasicJsonType` values. They accept every input `BasicJsonType::parse()` takes, and in
 addition a pointer to a null-terminated wide string (which `BasicJsonType::parse()` rejects). Contiguous inputs of
 single-byte characters that stay alive can be borrowed; all others (streams, `#!cpp FILE*`, wide characters, which
-must be converted to UTF-8, and non-contiguous iterators) are read into a copy that the document owns -- see the
+must be converted to UTF-8, and non-contiguous iterators) are read into a copy that the document owns — see the
 Notes. Use [`parse_copy()`](parse_copy.md) to get an owned copy of a borrowable input
 as well.
 
@@ -82,8 +82,8 @@ discarded; see [`is_discarded`](is_discarded.md).
 
 ## Exceptions
 
-Throws the same exception [`BasicJsonType::parse()`](../basic_json/parse.md) throws for the same input and options --
-the same exception id, message, and position -- because on a failing input the library's own parser is run on the
+Throws the same exception [`BasicJsonType::parse()`](../basic_json/parse.md) throws for the same input and options —
+the same exception id, message, and position — because on a failing input the library's own parser is run on the
 same bytes to produce the diagnostic. This replay runs only for a failing input, and costs about a lexing pass up to
 the error (its SAX handler builds no values); with `allow_exceptions` set to `#!cpp false`, it is skipped and a
 discarded document is returned instead. Additionally throws
@@ -100,7 +100,7 @@ Linear in the length of the input.
 
 | `input`                                                                             | ownership                                                    |
 |--------------------------------------------------------------------------------------|--------------------------------------------------------------|
-| lvalue byte container (`std::string`, `std::vector<char>`, ...), `std::string_view`, C string, character array | **borrowed** -- `input` must outlive the document |
+| lvalue byte container (`std::string`, `std::vector<char>`, ...), `std::string_view`, C string, character array | **borrowed** — `input` must outlive the document |
 | non-const rvalue `#!cpp std::string`                                                  | **owned**, moved in without a copy                            |
 | other rvalue byte container (including a `#!cpp const` rvalue `#!cpp std::string`)  | **owned**, copied                                             |
 | stream, `#!cpp FILE*`, wide characters, or anything else read through the general input adapter | **owned**, read into a buffer (a stream is read to its end) |
@@ -117,8 +117,8 @@ See [`owns_source`](owns_source.md) to check which happened after a call, and th
 **Numbers.** As for [`BasicJsonType::parse()`](../basic_json/parse.md), an integer literal too large for the 64-bit
 integer type becomes a floating-point value.
 
-**No lengths.** An integer argument that is not a `#!cpp bool` where the flags are expected -- for example
-`#!cpp parse(ptr, len)` -- does not compile (the overload is deleted). Such a call would convert `len` to
+**No lengths.** An integer argument that is not a `#!cpp bool` where the flags are expected — for example
+`#!cpp parse(ptr, len)` — does not compile (the overload is deleted). Such a call would convert `len` to
 `allow_exceptions` and read `ptr` as a null-terminated string, past the end of a buffer that has none. To parse a
 buffer of a given length, pass a pair of pointers: `#!cpp parse(ptr, ptr + len)`. The same holds for
 [`parse_copy`](parse_copy.md), [`accept`](accept.md), and [`read`](read.md).
@@ -151,11 +151,11 @@ buffer of a given length, pass a pair of pointers: `#!cpp parse(ptr, ptr + len)`
 
 ## See also
 
-- [parse_copy](parse_copy.md) - deserialize a copy of a compatible input
-- [accept](accept.md) - check whether the input is valid JSON
-- [read](read.md) - (re-)parse into this document, reusing its memory
-- [owns_source](owns_source.md) - return whether the document holds its own copy of the text
-- [load](load.md) - read a document from an image instead of parsing JSON text
+- [`parse_copy`](parse_copy.md) - deserialize a copy of a compatible input
+- [`accept`](accept.md) - check whether the input is valid JSON
+- [`read`](read.md) - (re-)parse into this document, reusing its memory
+- [`owns_source`](owns_source.md) - return whether the document holds its own copy of the text
+- [`load`](load.md) - read a document from an image instead of parsing JSON text
 - [`BasicJsonType::parse`](../basic_json/parse.md) - the corresponding function of `basic_json`
 
 ## Version history