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>

docs/mkdocs/docs/features/enum_conversion.md

@@ -29,6 +29,9 @@ The [`NLOHMANN_JSON_SERIALIZE_ENUM()` macro](../api/macros/nlohmann_json_seriali
 
 ## Usage
 
+Serialization converts an enum value to its mapped string, deserialization does the reverse, and an unrecognized JSON
+value deserializes to the first pair in the map:
+
 ```cpp
 // enum to JSON as string
 json j = TS_STOPPED;
@@ -43,6 +46,18 @@ json jPi = 3.14;
 assert(jPi.get<TaskState>() == TS_INVALID );
 ```
 
+??? example "Example: serializing/deserializing enums, including a second enum type"
+
+    ```cpp
+    --8<-- "examples/nlohmann_json_serialize_enum.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/nlohmann_json_serialize_enum.output"
+    ```
+
 ## Notes
 
 Just as in [Arbitrary Type Conversions](arbitrary_types.md) above,
@@ -54,9 +69,25 @@ Just as in [Arbitrary Type Conversions](arbitrary_types.md) above,
 
 Other Important points:
 
-- When using `get<ENUM_TYPE>()`, undefined JSON values will default to the first pair specified in your map. Select this
-  default pair carefully. If you desire an exception in this circumstance use [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()`](../api/macros/nlohmann_json_serialize_enum_strict.md)
-  which behaves identically except for throwing an exception on unrecognized values.
+- When using [`get<ENUM_TYPE>()`](../api/basic_json/get.md), undefined JSON values will default to the first pair
+  specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use
+  [`NLOHMANN_JSON_SERIALIZE_ENUM_STRICT()`](../api/macros/nlohmann_json_serialize_enum_strict.md) which behaves
+  identically except for throwing an
+  [`out_of_range.410`](../home/exceptions.md#jsonexceptionout_of_range410) exception on unrecognized values, both when
+  serializing an enum value not listed in the map and when deserializing a JSON value that matches none of the map's
+  entries.
 - If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the
   map will be returned when converting to or from JSON.
 - To disable the default serialization of enumerators as integers and force a compiler error instead, see [`JSON_DISABLE_ENUM_SERIALIZATION`](../api/macros/json_disable_enum_serialization.md).
+
+??? example "Example: `NLOHMANN_JSON_SERIALIZE_ENUM_STRICT` throwing on unrecognized values"
+
+    ```cpp
+    --8<-- "examples/nlohmann_json_serialize_enum_strict_err.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/nlohmann_json_serialize_enum_strict_err.output"
+    ```