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/binary_values.md

@@ -27,7 +27,7 @@ vector <|-- binary_t
 
 By default, binary values are stored as `std::vector<std::uint8_t>`. This type can be changed by providing a template
 parameter to the `basic_json` type. To store binary subtypes, the storage type is extended and exposed as
-`json::binary_t`:
+[`json::binary_t`](../api/basic_json/binary_t.md):
 
 ```cpp
 auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});
@@ -62,21 +62,23 @@ JSON values can be constructed from `json::binary_t`:
 json j = binary;
 ```
 
-Binary values are primitive values just like numbers or strings:
+Binary values are primitive values just like numbers or strings, as reflected by
+[`is_binary()`](../api/basic_json/is_binary.md) and [`is_primitive()`](../api/basic_json/is_primitive.md):
 
 ```cpp
 j.is_binary();    // returns true
 j.is_primitive(); // returns true
 ```
 
-Given a binary JSON value, the `binary_t` can be accessed by reference as via `get_binary()`:
+Given a binary JSON value, the `binary_t` can be accessed by reference via
+[`get_binary()`](../api/basic_json/get_binary.md):
 
 ```cpp
 j.get_binary().has_subtype();  // returns true
 j.get_binary().size();         // returns 4
 ```
 
-For convenience, binary JSON values can be constructed via `json::binary`:
+For convenience, binary JSON values can be constructed via [`json::binary`](../api/basic_json/binary.md):
 
 ```cpp
 auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);
@@ -99,7 +101,7 @@ JSON does not have a binary type, and this library does not introduce a new type
 Instead, binary values are serialized as an object with two keys: `bytes` holds an array of integers, and `subtype`
 is an integer or `null`.
 
-??? example
+??? example "Example: serialize a binary value to JSON"
 
     Code:
 
@@ -133,7 +135,7 @@ is an integer or `null`.
 [BJData](binary_formats/bjdata.md) neither supports binary values nor subtypes and proposes to serialize binary values
 as an array of uint8 values. The library implements this translation.
 
-??? example
+??? example "Example: serialize a binary value to BJData"
 
     Code:
     
@@ -192,7 +194,7 @@ as an array of uint8 values. The library implements this translation.
 [BON8](binary_formats/bon8.md) neither supports binary values nor subtypes. The library serializes binary values as an
 array of integers.
 
-??? example
+??? example "Example: serialize a binary value to BON8"
 
     Code:
 
@@ -227,7 +229,7 @@ array of integers.
 [BSON](binary_formats/bson.md) supports binary values and subtypes. If a subtype is given, it is used and added as an
 unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.
 
-??? example
+??? example "Example: serialize a binary value to BSON"
 
     Code:
     
@@ -269,7 +271,7 @@ unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00
 value will be serialized as byte strings. The library will choose the smallest representation using the length of the
 byte array.
 
-??? example
+??? example "Example: serialize a binary value to CBOR"
 
     Code:
     
@@ -294,7 +296,9 @@ byte array.
     ```
 
     Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless
-    `json::cbor_tag_handler_t::ignore` or `json::cbor_tag_handler_t::store` is passed to `json::from_cbor`.
+    `json::cbor_tag_handler_t::ignore` or `json::cbor_tag_handler_t::store` is passed to
+    [`json::from_cbor`](../api/basic_json/from_cbor.md) (see
+    [`cbor_tag_handler_t`](../api/basic_json/cbor_tag_handler_t.md)).
 
     ```json
     {
@@ -313,7 +317,7 @@ ext32. The subtype is then added as a signed 8-bit integer.
 
 If no subtype is given, the bin family (bin8, bin16, bin32) is used.
 
-??? example
+??? example "Example: serialize a binary value to MessagePack"
 
     Code:
     
@@ -353,7 +357,7 @@ If no subtype is given, the bin family (bin8, bin16, bin32) is used.
 [UBJSON](binary_formats/ubjson.md) neither supports binary values nor subtypes and proposes to serialize binary values
 as an array of uint8 values. The library implements this translation.
 
-??? example
+??? example "Example: serialize a binary value to UBJSON"
 
     Code: