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/get.md

@@ -13,16 +13,16 @@ BasicJsonType get() const;
 
 // (3)
 template<typename PointerType>
-PointerType get_ptr();
+PointerType get() noexcept;
 
 template<typename PointerType>
-constexpr const PointerType get_ptr() const noexcept;
+const PointerType get() const noexcept;  // constexpr since C++14
 ```
 
 1. Explicit type conversion between the JSON value and a compatible value which is
    [CopyConstructible](https://en.cppreference.com/w/cpp/named_req/CopyConstructible) and
    [DefaultConstructible](https://en.cppreference.com/w/cpp/named_req/DefaultConstructible). The value is converted by
-   calling the `json_serializer<ValueType>` `from_json()` method.
+   calling the [`json_serializer<ValueType>`](json_serializer.md) `from_json()` method.
    
     The function is equivalent to executing
     ```cpp
@@ -84,6 +84,12 @@ constexpr const PointerType get_ptr() const noexcept;
 3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; `#!cpp nullptr`
    otherwise
 
+## Exception safety
+
+Depends on what `json_serializer<ValueType>` `from_json()` method throws for overloads (1) and (2); the JSON value
+itself is never modified, since `get()` is a `#!cpp const` member function. No-throw guarantee for overload (3): this
+function never throws exceptions.
+
 ## Exceptions
 
 Depends on what `json_serializer<ValueType>` `from_json()` method throws
@@ -123,13 +129,13 @@ overload (3).
 
 ## Examples
 
-??? example
+??? example "Example: (1) explicit conversion to compatible types"
 
     The example below shows several conversions from JSON values
     to other types. There a few things to note: (1) Floating-point numbers can
     be converted to integers, (2) A JSON array can be converted to a standard
     `std::vector<short>`, (3) A JSON object can be converted to C++
-    associative containers such as `std::unordered_map<std::string, json>`.
+    associative containers such as `std::map<std::string, json>`.
         
     ```cpp
     --8<-- "examples/get__ValueType_const.cpp"
@@ -141,7 +147,21 @@ overload (3).
     --8<-- "examples/get__ValueType_const.output"
     ```
 
-??? example
+??? example "Example: (2) explicit conversion to another `basic_json` specialization"
+
+    The example below shows how a `json` value is converted to an `ordered_json` value using `get<BasicJsonType>()`.
+
+    ```cpp
+    --8<-- "examples/get__BasicJsonType.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/get__BasicJsonType.output"
+    ```
+
+??? example "Example: (3) explicit pointer access to the stored value"
 
     The example below shows how pointers to internal values of a JSON value can be requested. Note that no type
     conversions are made and a `#cpp nullptr` is returned if the value and the requested pointer type does not match.