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>

include/nlohmann/detail/json_pointer.hpp

@@ -77,7 +77,7 @@ class json_pointer
     }
 
     /// @brief return a string representation of the JSON pointer
-    /// @sa https://json.nlohmann.me/api/json_pointer/operator_string/
+    /// @sa https://json.nlohmann.me/api/json_pointer/operator_string_t/
     JSON_HEDLEY_DEPRECATED_FOR(3.11.0, to_string())
     operator string_t() const
     {
@@ -86,7 +86,7 @@ class json_pointer
 
 #ifndef JSON_NO_IO
     /// @brief write string representation of the JSON pointer to stream
-    /// @sa https://json.nlohmann.me/api/basic_json/operator_ltlt/
+    /// @sa https://json.nlohmann.me/api/operator_ltlt/
     friend std::ostream& operator<<(std::ostream& o, const json_pointer& ptr)
     {
         o << ptr.to_string();
@@ -1092,7 +1092,8 @@ class json_pointer
     friend bool operator!=(const StringType& lhs,
                            const json_pointer<RefStringTypeRhs>& rhs);
 
-    /// @brief compares two JSON pointer for less-than
+    /// @brief compares two JSON pointers for less-than
+    /// @sa https://json.nlohmann.me/api/json_pointer/operator_spaceship/
     template<typename RefStringTypeLhs, typename RefStringTypeRhs>
     // NOLINTNEXTLINE(readability-redundant-declaration)
     friend bool operator<(const json_pointer<RefStringTypeLhs>& lhs,