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/scripts/slideshow/README.md

@@ -0,0 +1,27 @@
+# Feature slideshow of the home page
+
+`make` builds `docs/images/json-light.svg` and `json-dark.svg`, the animated feature tour on the home page. They replace
+`json.gif` there. All text is outlined, so the SVGs need no fonts. Material's theme toggle cannot be seen from an
+`<img>`, so the home page includes both files (`#only-light`, `#only-dark`) and Material shows the one that matches.
+
+The slides are in [`slides.md`](slides.md): one `# Title` per slide (`{badge: TEXT}` at the end adds a red chip), `- `
+bullets, and one fenced code block. `build.py` adds the title and closing slides around them, so the slide count follows
+the file. Every snippet should compile against the current `develop`. Use a non-breaking space to keep words together
+("16 bytes").
+
+```bash
+make          # download missing fonts, build both SVGs
+make SECONDS=12
+make clean    # remove the SVGs; make distclean also removes the fonts
+```
+
+Run it from this directory. It needs [uv](https://docs.astral.sh/uv/), plus curl, unzip and shasum for the fonts:
+`build.py` declares its dependencies inline (PEP 723, pinned, so the output stays byte-identical) and `uv run --script`
+provides them. `fonts/` is git-ignored: Inter (variable, Google Fonts repository) and JetBrains Mono 2.304 (variable)
+are downloaded from pinned URLs and checked against their SHA-256; a changed upstream file stops the build.
+`./build.py OUT.svg [seconds] [auto|light|dark]` works directly too. A mistake in `slides.md` stops the build with
+`slides.md:LINE` and leaves the old SVGs in place.
+
+Layout: title 48 px, bullets 24 px, code 19 px; the build warns when a slide's code would drop below 18 px. Syntax colours
+are the `code-*` tokens of the brand (also used by `css/brand.css`). A CSS animation dissolves each slide over the
+previous one and fills the progress bar; with reduced motion only the first slide shows.