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/docs/features/binary_formats/messagepack.md

@@ -79,7 +79,7 @@ specification:
     total), because the check used to select the smaller float 32 encoding compared magnitudes with NaN, which is
     always `false` and caused the float 32 path to be skipped.
 
-??? example
+??? example "Example: serialize a JSON value to MessagePack"
 
     ```cpp
     --8<-- "examples/to_msgpack.cpp"
@@ -162,7 +162,7 @@ The library maps MessagePack types to JSON value types as follows:
     value is dumped. `bin`/`ext`/`fixext` values are unaffected and are never validated, since they are not required
     to hold text.
 
-??? example
+??? example "Example: deserialize a JSON value from MessagePack"
 
     ```cpp
     --8<-- "examples/from_msgpack.cpp"