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/Makefile

@@ -35,9 +35,10 @@ create_output: $(EXAMPLES:.cpp=.output)
 # check output of all stand-alone example files
 check_output: $(EXAMPLES:.cpp=.test)
 
-# check output of all stand-alone example files (exclude files with platform-dependent output.)
+# check output of all stand-alone example files (exclude files whose output depends on the platform by nature:
+# library and compiler information, container size limits, and hash values)
 # This target is used in the CI (ci_test_documentation).
-check_output_portable: $(filter-out mkdocs/docs/examples/meta.test mkdocs/docs/examples/max_size.test mkdocs/docs/examples/std_hash.test mkdocs/docs/examples/basic_json__CompatibleType.test,$(EXAMPLES:.cpp=.test))
+check_output_portable: $(filter-out mkdocs/docs/examples/meta.test mkdocs/docs/examples/max_size.test mkdocs/docs/examples/std_hash.test,$(EXAMPLES:.cpp=.test))
 
 clean:
 	rm -fr $(EXAMPLES:.cpp=)