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/images/memory_over_time.svg

@@ -0,0 +1,20 @@
+<svg class="nj-fig nj-fig--sax-mem" viewBox="0 0 660 312" role="img" aria-labelledby="fig-sax-mem-title fig-sax-mem-desc" xmlns="http://www.w3.org/2000/svg">
+<title id="fig-sax-mem-title">Illustrative memory use over time while a large array of records is parsed, not a measurement.</title>
+<desc id="fig-sax-mem-desc">Four curves. D&#8203;OM, parse(): rises steadily until the whole tree is in memory. json_document: starts at about the size of the text, which it keeps, and grows a little while its index is built. Parser callback that discards every record: stays flat at the size of one record. S&#8203;AX: stays flat and lowest, because no value is built and only what the handler keeps is in memory.</desc>
+<path class="nj-sax-axis" d="M56 36 V272 H448"/>
+<text class="fg-h" x="16" y="24">memory</text>
+<text class="fg-note fg-note--l" x="56" y="294">time: records parsed</text>
+<text class="fg-note fg-note--r nj-sax-illus" x="648" y="24">illustrative, not a measurement</text>
+<path class="nj-sax-curve nj-sax-c-dom" d="M56 268 L440 52"/>
+<path class="nj-sax-curve nj-sax-c-doc" d="M56 168 L440 146"/>
+<path class="nj-sax-curve nj-sax-c-cb" d="M56 250 L88 238 L88 250 L120 238 L120 250 L152 238 L152 250 L184 238 L184 250 L216 238 L216 250 L248 238 L248 250 L280 238 L280 250 L312 238 L312 250 L344 238 L344 250 L376 238 L376 250 L408 238 L408 250 L440 238 L440 250"/>
+<path class="nj-sax-curve nj-sax-c-sax" d="M56 264 H440"/>
+<text class="nj-sax-lab nj-sax-c-dom" x="456" y="48">D&#8203;OM: parse()</text>
+<text class="fg-note fg-note--l" x="456" y="65">the whole tree</text>
+<text class="nj-sax-lab nj-sax-c-doc" x="456" y="142">json_document</text>
+<text class="fg-note fg-note--l" x="456" y="159">the text plus its index</text>
+<text class="nj-sax-lab nj-sax-c-cb" x="456" y="227">callback, discard</text>
+<text class="fg-note fg-note--l" x="456" y="244">one record at a time</text>
+<text class="nj-sax-lab nj-sax-c-sax" x="456" y="269">S&#8203;AX</text>
+<text class="fg-note fg-note--l" x="456" y="286">only what the handler keeps</text>
+</svg>