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/save.md

@@ -6,13 +6,13 @@ std::vector<std::uint8_t> save() const;
 
 Writes the document as an *image*: a byte buffer that [`load`](load.md) reads back without parsing. The image holds
 the node index, the source text (plus, for an edited document, the number tokens edits wrote), and the decoded
-strings (plus the strings edits wrote) -- everything [`root()`](root.md) needs, with nothing left to parse.
+strings (plus the strings edits wrote) — everything [`root()`](root.md) needs, with nothing left to parse.
 
 An edited document is written in its *current* state, with its values in document order, the way the library's own
 parser would have produced them for that JSON text: a member [`set`](set.md) added goes at the end, an
 [`erase`](erase.md)d member leaves no trace, and a float that is not finite (NaN or positive/negative infinity)
 becomes null, the same substitution [`dump()`](../basic_json_view/dump.md) makes. The same document always saves to
-the same bytes -- also across `BasicJsonType` and the [`view_mode`](../view_mode.md), since the image reflects document order and
+the same bytes — also across `BasicJsonType` and the [`view_mode`](../view_mode.md), since the image reflects document order and
 values only, not which specialization produced them.
 
 ## Return value
@@ -28,11 +28,11 @@ exactly as it was, and the partially built image is discarded with the exception
 ## Exceptions
 
 Throws [`type_error.320`](../../home/exceptions.md#jsonexceptiontype_error320) if the document is
-[discarded](is_discarded.md) -- a default-constructed document, or one a failed [`parse()`](parse.md)/
+[discarded](is_discarded.md) — a default-constructed document, or one a failed [`parse()`](parse.md)/
 [`read()`](read.md) with `allow_exceptions == false` left discarded.
 
 Throws [`out_of_range.416`](../../home/exceptions.md#jsonexceptionout_of_range416) if the node count, the text, or
-the decoded strings of the image would individually reach 4 GiB -- the same 32-bit offsets
+the decoded strings of the image would individually reach 4 GiB — the same 32-bit offsets
 [`parse()`](parse.md#exceptions) and, for edits, [`set`](set.md)/[`push_back`](push_back.md) are already limited to.
 
 !!! failure "Example messages"
@@ -55,13 +55,13 @@ up in the image.
 and the sizes of the text and the decoded strings, all little-endian), followed by the nodes
 ([16 bytes each](../../home/architecture.md#node-index-of-json-views)), the text and a `#!cpp '\0'`, and the decoded
 strings and a `#!cpp '\0'`. [`load`](load.md) checks the header, and the sizes it describes, before reading anything
-else -- see [`load`'s Exceptions](load.md#exceptions).
+else — see [`load`'s Exceptions](load.md#exceptions).
 
 !!! warning "Experimental"
 
     The image format is versioned but not yet stable: it may change in an incompatible way before it is declared
     stable. Use images to cache a document within one build of the library, or to hand one to another process running
-    the *same* build on the *same* (little-endian) machine -- not as a long-term storage format. Keep the original
+    the *same* build on the *same* (little-endian) machine — not as a long-term storage format. Keep the original
     JSON text if you need to read a saved document back with a future library version.
 
 **Little-endian targets only.** The image is written as raw little-endian bytes, with no byte-swapping. On a
@@ -72,8 +72,8 @@ big-endian reader could not interpret correctly.
 
 ??? example "Caching a parsed document as an image"
 
-    The example below saves a parsed configuration as an image -- the way a service might cache one to answer later
-    requests without parsing the text again -- and confirms that loading it back gives exactly the same result as
+    The example below saves a parsed configuration as an image — the way a service might cache one to answer later
+    requests without parsing the text again — and confirms that loading it back gives exactly the same result as
     parsing did, and that saving is deterministic.
 
     ```cpp
@@ -88,8 +88,8 @@ big-endian reader could not interpret correctly.
 
 ## See also
 
-- [load](load.md) - read an image written by `save()`
-- [owns_source](owns_source.md) - return whether the document holds its own copy of the text
+- [`load`](load.md) - read an image written by `save()`
+- [`owns_source`](owns_source.md) - return whether the document holds its own copy of the text
 - [`basic_json_view::dump`](../basic_json_view/dump.md) - serialize the document to JSON text instead of an image
 - [Images](../../features/json_view.md#images) - why and when to use images