Review and extend the documentation, and check it in CI

A review of all documentation pages found factual errors, dead links,
missing cross-references, and gaps in examples. This fixes them and adds
checks so the same problems are caught automatically.

Fixes:
- wrong signatures and version histories (operator!= C++20 member,
  binary() subtype type, get<PointerType>(), JSON_NO_THREAD_LOCAL, ...)
- stale descriptions (number parsing since #5283, UBJSON table, SAX
  example that no longer compiled, tsl::ordered_map advice)
- dead internal and external links; repology.org badges (the domain is
  suspended) replaced by badges that query the registries directly
- deprecation notes link the migration guide; the guide itself fixed

Additions:
- "See also" sections, cross-references, 25 runnable examples, 12
  Mermaid diagrams, new API pages for json_pointer::operator<=> and
  byte_container_with_subtype::operator==/!=
- landing page, guides for untrusted input and performance
- "unreleased" badge after versions newer than the latest release

Checks:
- strict documentation build (broken links/anchors fail it); CI and
  the publish workflow fetch the full history the build needs
- weekly external link check, Mermaid syntax check in CI
- check_structure.py: example titles, heading levels, alt texts,
  header links, docset index coverage; its unused-example check works
  again
- all examples produce the same output on every platform

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/Makefile

@@ -1,3 +1,8 @@
+# The MkDocs 2.0 banners of Material for MkDocs and ProperDocs (via mkdocs-redirects) are printed to stderr, not
+# logged, so they do not affect --strict; silence them anyway.
+export NO_MKDOCS_2_WARNING := true
+export DISABLE_MKDOCS_2_WARNING := true
+
 # serve the site locally
 serve: style_check
 	venv/bin/mkdocs serve
@@ -8,11 +13,17 @@ serve_dirty: style_check
 # This target is used in the CI (ci_test_build_documentation).
 # This target is used by the docset Makefile.
 build: style_check
-	venv/bin/mkdocs build
+	venv/bin/mkdocs build --strict
 
 style_check:
 	@cd docs ; ../venv/bin/python3 ../scripts/check_structure.py
 
+# check that all Mermaid diagrams parse (needs Node.js)
+# This target is used in the CI (ci_test_documentation_mermaid).
+check_mermaid:
+	npm ci --prefix scripts/mermaid --ignore-scripts --no-audit --no-fund
+	node scripts/mermaid/check_mermaid.mjs docs
+
 # check the links in the documentation files in docs/mkdocs
 link_check:
 	ENABLED_HTMLPROOFER=true venv/bin/mkdocs build