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/dump.md

@@ -11,7 +11,7 @@ Serializes this value (and its subtree) directly from the flat index, without ev
 first. With the default `#!cpp numbers == number_format::shortest`, the result is the same string
 [`BasicJsonType::dump`](../basic_json/dump.md) would produce for the value
 [`BasicJsonType::parse()`](../basic_json/parse.md) builds from the same source text, called with the same `indent`,
-`indent_char`, and `ensure_ascii` -- except that members of an object appear in document order rather than sorted by
+`indent_char`, and `ensure_ascii` — except that members of an object appear in document order rather than sorted by
 key, and *every* occurrence of a repeated key is written rather than only the last one (see
 [Notes on duplicate keys](operator[].md#notes)). For a `json_view` (whose `BasicJsonType` is not ordered), this means
 `dump()` can print an object's members in a different order than [`materialize()`](materialize.md)`.dump()` of the
@@ -58,19 +58,19 @@ Linear in the size of the output text.
 ## Notes
 
 The walk over the subtree is iterative, so the nesting depth it can write is limited by available memory only, not by
-the call stack -- as for [`materialize()`](materialize.md).
+the call stack — as for [`materialize()`](materialize.md).
 
 Strings are escaped by the same rules as [`BasicJsonType::dump`](../basic_json/dump.md).
 
 With `#!cpp numbers == number_format::shortest`, floats are written with the library's shortest round-trip conversion,
 exactly as [`BasicJsonType::dump`](../basic_json/dump.md) would (e.g. `#!cpp 1.5`, `#!cpp 100.0`, `#!cpp 1e+100`), and
-integers are copied from the source text -- already canonical in JSON, so this matches their shortest form too --
+integers are copied from the source text — already canonical in JSON, so this matches their shortest form too —
 except that `#!cpp -0` is written as `#!cpp 0`, the way [`BasicJsonType::parse()`](../basic_json/parse.md) reads it.
 
 `#!cpp number_format::source` copies every number exactly as written in the source text instead, with no exception
-for `#!cpp -0` -- `#!cpp 1.50`, `#!cpp 1E2`, `#!cpp -0.0`, `#!cpp -0`, or all digits of an integer literal with more
+for `#!cpp -0` — `#!cpp 1.50`, `#!cpp 1E2`, `#!cpp -0.0`, `#!cpp -0`, or all digits of an integer literal with more
 digits than any number type holds (such a literal is itself classified as a float, see
-[What is different](../../features/json_view.md#what-is-different)) -- something `BasicJsonType` cannot do, since
+[What is different](../../features/json_view.md#what-is-different)) — something `BasicJsonType` cannot do, since
 parsing already reduces every number to its parsed value.
 
 ## Examples
@@ -95,9 +95,10 @@ parsing already reduces every number to its parsed value.
 ## See also
 
 - [`number_format`](number_format.md) - how `dump()` writes numbers
-- [operator<<](operator_ltlt.md) - serialize this value to a stream
-- [materialize](materialize.md) - build a `BasicJsonType` value, e.g. to use `BasicJsonType::dump`'s `error_handler`
+- [`operator<<`](operator_ltlt.md) - serialize this value to a stream
+- [`materialize`](materialize.md) - build a `BasicJsonType` value, e.g. to use `BasicJsonType::dump`'s `error_handler`
 - [`BasicJsonType::dump`](../basic_json/dump.md) - the corresponding function of `basic_json`
+- [Zero-copy JSON views](../../features/json_view.md#writing-a-view-back) - writing a view back without building a tree
 
 ## Version history