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_string.md

@@ -41,8 +41,8 @@ counterpart, except that it returns a view instead of a reference to a value tha
 document, the returned [`string_view_t`](index.md#member-types) is valid as long as the caller's buffer; for an owned
 one, as long as the document.
 
-The returned [`string_view_t`](index.md#member-types) is valid exactly as long as the view that produced it -- see the
-[validity rules](index.md) of `basic_json_view` -- and, for a string with no escapes, for as long as the document's
+The returned [`string_view_t`](index.md#member-types) is valid exactly as long as the view that produced it — see the
+[validity rules](index.md) of `basic_json_view` — and, for a string with no escapes, for as long as the document's
 source text, which for a borrowed document must remain valid **and unmodified** for as long as the document exists,
 since floating-point values and unescaped strings are read from the source lazily.
 
@@ -52,8 +52,8 @@ since floating-point values and unescaped strings are read from the source lazil
 
     The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no
     `#!cpp std::string` was allocated for it: the returned view still points inside the original buffer. A field that
-    contains an escape sequence cannot point into the original text -- it was decoded once into the document's own
-    buffer instead -- but still avoids a per-field allocation.
+    contains an escape sequence cannot point into the original text — it was decoded once into the document's own
+    buffer instead — but still avoids a per-field allocation.
 
     ```cpp
     --8<-- "examples/basic_json_view__get_string.cpp"
@@ -67,9 +67,10 @@ since floating-point values and unescaped strings are read from the source lazil
 
 ## See also
 
-- [get](get.md) - convert the value to a given type (`#!cpp get<string_view_t>()` is equivalent to this function)
-- [number_token](number_token.md) - a number's token text, without a copy
+- [`get`](get.md) - convert the value to a given type (`#!cpp get<string_view_t>()` is equivalent to this function)
+- [`number_token`](number_token.md) - a number's token text, without a copy
 - [`BasicJsonType::get_ref`](../basic_json/get_ref.md) - the closest counterpart of `basic_json`
+- [Zero-copy JSON views](../../features/json_view.md#getting-values-out-without-copying) - borrowing strings instead of copying them
 
 ## Version history