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/object_t.md

@@ -33,7 +33,7 @@ To store objects in C++, a type is defined by the template parameters described
 
 ## Notes
 
-#### Default type
+### Default type
 
 With the default values for `ObjectType` (`std::map`), `StringType` (`std::string`), and `AllocatorType`
 (`std::allocator`), the default value for `object_t` is:
@@ -58,7 +58,7 @@ std::map<
 
 See [`default_object_comparator_t`](default_object_comparator_t.md) for more information.
 
-#### Behavior
+### Behavior
 
 The choice of `object_t` influences the behavior of the JSON class. With the default type, objects have the following
 behavior:
@@ -76,7 +76,7 @@ behavior:
   that they will not be affected by these differences. For instance, `#!json {"b": 1, "a": 2}` and
   `#!json {"a": 2, "b": 1}` will be treated as equal.
 
-#### Limits
+### Limits
 
 [RFC 8259](https://tools.ietf.org/html/rfc8259) specifies:
 > An implementation may set limits on the maximum depth of nesting.
@@ -85,12 +85,12 @@ In this class, the object's limit of nesting is not explicitly constrained. Howe
 introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the
 [`max_size`](max_size.md) function of a JSON object.
 
-#### Storage
+### Storage
 
 Objects are stored as pointers in a `basic_json` type. That is, for any access to object values, a pointer of type
 `object_t*` must be dereferenced.
 
-#### Object key order
+### Object key order
 
 The order name/value pairs are added to the object are *not* preserved by the library. Therefore, iterating an object
 may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in
@@ -98,10 +98,10 @@ alphabetical order as `std::map` with `std::less` is used by default. Please not
 [RFC 8259](https://tools.ietf.org/html/rfc8259), because any order implements the specified "unordered" nature of JSON
 objects.
 
-#### Cross-`basic_json` conversion requirements
+### Cross-`basic_json` conversion requirements
 
 When converting an object from one `basic_json` specialization to another via the
-[converting constructor](basic_json.md#overload-4), the target `object_t`'s `key_type` must be
+[converting constructor](basic_json.md) (overload 4), the target `object_t`'s `key_type` must be
 directly constructible from the source `basic_json`'s `string_t` type (or more generally, from the
 source object's key type). If this requirement is not met, the conversion does not fail; instead,
 the object is silently converted as an array of key-value pairs, which is incorrect. See
@@ -123,6 +123,13 @@ the object is silently converted as an array of key-value pairs, which is incorr
     --8<-- "examples/object_t.output"
     ```
 
+## See also
+
+- [array_t](array_t.md) the type used to store JSON arrays
+- [string_t](string_t.md) the type used to store JSON strings
+- [object_comparator_t](object_comparator_t.md) the comparator used to order object keys
+- [Object Order](../../features/object_order.md) - the article on object key ordering
+
 ## Version history
 
 - Added in version 1.0.0.