Review and extend the documentation, and check it in CI (#5638)

* 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>

* Keep the customer links that could not be fixed

A dead link on the customers page is still the evidence of where the
use of the library was documented. Keep the original URLs of the entries
without a working replacement (Marne, Cisco Webex Desk Camera, Philips
Hue, CyberArk) and exclude exactly these URLs from the link check.

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

* Correct the duplicate-key recipe's claim about SAX positions

The SAX interface's key() receives no position either; only parse_error()
does. Also note that the recipe does not report the path to the repeated
key (see discussion #5085).

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

* Say the library is available as a single header and mention json_fwd.hpp

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

* Correct documentation errors found while hunting for bugs

- patch/patch_inplace: list the JSON pointer errors parse_error.106-109
  and out_of_range.402/404, and quote the actual parse_error.105 message.
- unflatten: list parse_error.106/107/108 and out_of_range.404.
- to_bson: list out_of_range.415 (binary subtype above 255) and note
  that 412 and 415 are new in 3.13.0.
- to_string: state that string_t must be convertible to std::string, also
  in the StringType requirements table.
- JSON Lines: a `while (input >> j)` loop also throws after the last value
  for concatenated JSON values; show a loop that works for both.
- BON8: a string gets 0xFF only if nothing follows it in the message; a
  string at the end of an array or object is ended by 0xFE.
- custom_string_type.hpp: add operator+=(char), which the "Always
  required" list asks for (json_pointer::to_string, flatten, unflatten,
  and diff did not compile), and an ADL int_to_string for diff and items.

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

* Cache the release headers with functools.lru_cache

Codacy (Pylint) flagged the mutable default argument that header() used
as its cache. functools.lru_cache keeps the same memoization without it.
The script's output is unchanged.

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

---------

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

single_include/nlohmann/json.hpp

@@ -2710,7 +2710,7 @@ void templated_json_throw(ExceptionType exception)
 @brief macro to briefly define a mapping between an enum and JSON with exception
        on invalid input
 @def NLOHMANN_JSON_SERIALIZE_ENUM_STRICT
-@since version 3.12.0
+@since version 3.13.0
 */
 #define NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(ENUM_TYPE, ...)                                     \
     template<typename BasicJsonType>                                                            \
@@ -17904,8 +17904,9 @@ using parser_callback_t =
 /*!
 @brief syntax analysis
 
-This class implements an iterative parser that keeps the open containers on
-an explicit stack and reports what it reads as SAX events.
+This class implements a parser for JSON text. Nested arrays and objects are tracked with an explicit
+stack instead of recursion, so deeply nested input does not exhaust the call stack, and what is read
+is reported as SAX events.
 */
 template<typename BasicJsonType, typename InputAdapterType>
 class parser
@@ -19646,7 +19647,7 @@ 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
     {
@@ -19655,7 +19656,7 @@ 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();
@@ -20667,7 +20668,8 @@ 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,
@@ -31825,7 +31827,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     /// @{
 #ifndef JSON_NO_IO
     /// @brief serialize 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 basic_json& j)
     {
         // read width member and use it as the indentation parameter if nonzero
@@ -31844,7 +31846,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     }
 
     /// @brief serialize to stream
-    /// @sa https://json.nlohmann.me/api/basic_json/operator_ltlt/
+    /// @sa https://json.nlohmann.me/api/operator_ltlt/
     /// @deprecated This function is deprecated since 3.0.0 and will be removed in
     ///             version 4.0.0 of the library. Please use
     ///             operator<<(std::ostream&, const basic_json&) instead; that is,
@@ -32013,7 +32015,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
 #endif
 #ifndef JSON_NO_IO
     /// @brief deserialize from stream
-    /// @sa https://json.nlohmann.me/api/basic_json/operator_gtgt/
+    /// @sa https://json.nlohmann.me/api/operator_gtgt/
     /// @deprecated This stream operator is deprecated since 3.0.0 and will be removed in
     ///             version 4.0.0 of the library. Please use
     ///             operator>>(std::istream&, basic_json&) instead; that is,
@@ -32025,7 +32027,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
     }
 
     /// @brief deserialize from stream
-    /// @sa https://json.nlohmann.me/api/basic_json/operator_gtgt/
+    /// @sa https://json.nlohmann.me/api/operator_gtgt/
     friend std::istream& operator>>(std::istream& i, basic_json& j)
     {
         // parse into a temporary so that j is left unchanged if parsing fails