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/json_pointer/index.md

@@ -20,6 +20,24 @@ are the base for JSON patches.
     in which case `string_t` will be deduced as [`basic_json::string_t`](../basic_json/string_t.md). This feature is
     deprecated and may be removed in a future major version.
 
+    See the [migration guide](../../integration/migration_guide.md#json-pointers) for how to update existing code.
+
+A JSON pointer is internally a sequence of reference tokens. [`front`](front.md), [`pop_front`](pop_front.md), and
+[`push_front`](push_front.md) act on the first reference token, whereas [`back`](back.md), [`pop_back`](pop_back.md),
+and [`push_back`](push_back.md) act on the last one. [`parent_pointer`](parent_pointer.md) returns a new JSON pointer
+with the last reference token removed (like a non-mutating [`pop_back`](pop_back.md)):
+
+```mermaid
+flowchart LR
+    A["a"] --> B["b"] --> C["c"]
+
+    front["front() / pop_front() / push_front()"] -.-> A
+    back["back() / pop_back() / push_back()"] -.-> C
+    parent["parent_pointer() returns /a/b"] -.-> B
+```
+
+The diagram shows the reference tokens of the JSON pointer `/a/b/c`.
+
 ## Member types
 
 - [**string_t**](string_t.md) - the string type used for the reference tokens
@@ -28,9 +46,10 @@ are the base for JSON patches.
 
 - [(constructor)](json_pointer.md)
 - [**to_string**](to_string.md) - return a string representation of the JSON pointer
-- [**operator string_t**](operator_string_t.md) - return a string representation of the JSON pointer
+- [**operator string_t**](operator_string_t.md) - return a string representation of the JSON pointer (deprecated)
 - [**operator==**](operator_eq.md) - compare: equal
 - [**operator!=**](operator_ne.md) - compare: not equal
+- [**operator<=>**](operator_spaceship.md) - compare: 3-way (C++20)
 - [**operator/=**](operator_slasheq.md) - append to the end of the JSON pointer
 - [**operator/**](operator_slash.md) - create JSON Pointer by appending
 - [**parent_pointer**](parent_pointer.md) - returns the parent of this JSON pointer
@@ -45,6 +64,7 @@ are the base for JSON patches.
 ## Literals
 
 - [**operator""_json_pointer**](../operator_literal_json_pointer.md) - user-defined string literal for JSON pointers
+
 ## See also
 
 - [RFC 6901](https://datatracker.ietf.org/doc/html/rfc6901)