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/api/basic_json/string_t.md

@@ -30,16 +30,16 @@ JSON class into byte-sized characters during deserialization.
 
 ## Notes
 
-#### Default type
+### Default type
 
 With the default values for `StringType` (`std::string`), the default value for `string_t` is `#!cpp std::string`.
 
-#### Encoding
+### Encoding
 
 Strings are stored in UTF-8 encoding. Therefore, functions like `std::string::size()` or `std::string::length()` return
 the number of bytes in the string rather than the number of characters or glyphs.
 
-#### String comparison
+### String comparison
 
 [RFC 8259](https://tools.ietf.org/html/rfc8259) states:
 > Software implementations are typically required to test names of object members for equality. Implementations that
@@ -50,15 +50,15 @@ the number of bytes in the string rather than the number of characters or glyphs
 
 This implementation is interoperable as it does compare strings code unit by code unit.
 
-#### Storage
+### Storage
 
 String values are stored as pointers in a `basic_json` type. That is, for any access to string values, a pointer of type
 `string_t*` must be dereferenced.
 
-#### Cross-`basic_json` conversion requirements
+### Cross-`basic_json` conversion requirements
 
 When converting a string value from one `basic_json` specialization to another via the
-[converting constructor](basic_json.md#overload-4), the target `string_t` must be directly
+[converting constructor](basic_json.md) (overload 4), the target `string_t` must be directly
 constructible from the source `basic_json`'s `string_t` type. If this requirement is not met, the
 conversion does not fail; instead, the string is silently converted as an array of character codes,
 which is incorrect. See [issue #3425](https://github.com/nlohmann/json/issues/3425) for details
@@ -80,6 +80,12 @@ and an example.
     --8<-- "examples/string_t.output"
     ```
 
+## See also
+
+- [object_t](object_t.md) the type used to store JSON objects (and their keys, which are also `string_t`)
+- [binary_t](binary_t.md) the type used to store binary values
+- [get_ptr](get_ptr.md) returns a pointer to the stored string value
+
 ## Version history
 
 - Added in version 1.0.0.