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/patch_inplace.md

@@ -22,10 +22,24 @@ No guarantees, value may be corrupted by an unsuccessful patch operation.
 - Throws [`parse_error.104`](../../home/exceptions.md#jsonexceptionparse_error104) if the JSON patch does not consist of
   an array of objects.
 - Throws [`parse_error.105`](../../home/exceptions.md#jsonexceptionparse_error105) if the JSON patch is malformed (e.g.,
-  mandatory attributes are missing); example: `"operation add must have member path"`.
+  mandatory attributes are missing); example: `"operation 'add' must have member 'path'"`.
 - Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if an array index is out of range.
+- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in a "path" or
+  "from" member begins with '0'; example: `"array index '01' must not begin with '0'"`.
+- Throws [`parse_error.107`](../../home/exceptions.md#jsonexceptionparse_error107) if a "path" or "from" member is not
+  empty and does not begin with a slash (`/`); example: `"JSON pointer must be empty or begin with '/' - was: 'a'"`.
+- Throws [`parse_error.108`](../../home/exceptions.md#jsonexceptionparse_error108) if a tilde (`~`) in a "path" or
+  "from" member is not followed by `0` or `1`; example: `"escape character '~' must be followed with '0' or '1'"`.
+- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in a "path" or
+  "from" member is not a number; example: `"array index 'foo' is not a number"`.
+- Throws [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if the array index `-` is used
+  where an existing element is required (the "path" of "replace", the "from" of "move" and "copy"); example:
+  `"array index '-' (3) is out of range"`.
 - Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if a JSON pointer inside the patch
   could not be resolved successfully in the current JSON value; example: `"key baz not found"`.
+- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if a reference token of a JSON
+  pointer inside the patch cannot be resolved, e.g., `-` in a "remove" operation or `1a` for an array; example:
+  `"unresolved reference token '-'"`.
 - Throws [`out_of_range.405`](../../home/exceptions.md#jsonexceptionout_of_range405) if JSON pointer has no parent
   ("add", "remove", "move")
 - Throws [`out_of_range.411`](../../home/exceptions.md#jsonexceptionout_of_range411) if an "add" operation's target
@@ -50,7 +64,7 @@ function throws an exception.
 
 ## Examples
 
-??? example
+??? example "Example: apply a JSON patch in place"
 
     The following code shows how a JSON patch is applied to a value.
      
@@ -64,6 +78,22 @@ function throws an exception.
     --8<-- "examples/patch_inplace.output"
     ```
 
+??? example "Example: out_of_range.403 exception with a partially applied patch"
+
+    The following code shows a patch whose first operation succeeds and whose second operation fails. Because
+    `patch_inplace` applies each operation directly to the value, the first operation's effect is still visible after
+    the exception is caught, unlike [`patch`](patch.md).
+
+    ```cpp
+    --8<-- "examples/patch_inplace__exception.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/patch_inplace__exception.output"
+    ```
+
 ## See also
 
 - [RFC 6902 (JSON Patch)](https://tools.ietf.org/html/rfc6902)