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/basic_json.md

@@ -99,6 +99,25 @@ basic_json(basic_json&& other) noexcept;
       elements of the pairs are treated as keys and the second elements are as values.
     3. In all other cases, an array is created.
     
+    The following flowchart also takes into account what happens when `type_deduction` is `#!cpp false`, in which case
+    `manual_type` decides between object and array, and an object can only be forced if `init` actually matches rule 2
+    (or is empty):
+
+    ```mermaid
+    flowchart TD
+        A(["initializer_list init"]) --> B{"empty, or every element is a 2-element<br/>array whose first element is a string?"}
+        B -->|"yes"| C{"type_deduction"}
+        B -->|"no"| D{"type_deduction"}
+        C -->|"true"| OBJ["create object"]
+        C -->|"false"| E{"manual_type"}
+        E -->|"object"| OBJ
+        E -->|"array"| ARR["create array"]
+        D -->|"true"| ARR
+        D -->|"false"| F{"manual_type"}
+        F -->|"array"| ARR
+        F -->|"object"| ERR["throw type_error.301"]
+    ```
+
     The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows:
     
     1. The empty initializer list is written as `#!cpp {}` which is exactly an empty JSON object.
@@ -171,8 +190,8 @@ basic_json(basic_json&& other) noexcept;
     - `BasicJsonType` has different template arguments than `basic_json_t`.
     
     **Note:** For cross-`basic_json` conversions to produce correct results, the target `basic_json`'s
-    `object_t::key_type` and `string_t` must be directly constructible from the source `basic_json`'s
-    corresponding types. See the description of overload (4) above for details on what happens when
+    [`object_t`](object_t.md)`::key_type` and [`string_t`](string_t.md) must be directly constructible from the source
+    `basic_json`'s corresponding types. See the description of overload (4) above for details on what happens when
     this requirement is not met.
 
 `U`:
@@ -347,6 +366,22 @@ basic_json(basic_json&& other) noexcept;
 
     Note the output is platform-dependent.
 
+??? example "Example: (4) create a JSON value from another `basic_json` specialization"
+
+    The example below shows how a `json` value is converted to an `ordered_json` value and back using the converting
+    constructor. Note how the original insertion order of `oj` is not restored, because it was already given up when
+    converting to `json`, whose `object_t` sorts by key.
+
+    ```cpp
+    --8<-- "examples/basic_json__BasicJsonType.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json__BasicJsonType.output"
+    ```
+
 ??? example "Example: (5) create a container (array or object) from an initializer list"
 
     The example below shows how JSON values are created from initializer lists.
@@ -417,6 +452,15 @@ basic_json(basic_json&& other) noexcept;
     --8<-- "examples/basic_json__moveconstructor.output"
     ```
 
+## See also
+
+- [array](array.md) create a JSON array value, forcing array creation from an initializer list even when it looks like
+  an object
+- [object](object.md) create a JSON object value, forcing object creation from an initializer list
+- [binary](binary.md) create a JSON binary array value
+- [operator=](operator=.md) copy assignment operator
+- [Creating JSON values](../../features/creating_values.md) - the article on creating JSON values
+
 ## Version history
 
 1. Since version 1.0.0.