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/integration/migration_guide.md

@@ -1,6 +1,8 @@
 # Migration Guide
 
-This page collects some guidelines on how to future-proof your code for future versions of this library.
+This page collects some guidelines on how to future-proof your code for future versions of this library. For how to
+add the library to your project in the first place, see [Integration](index.md), [CMake](cmake.md), or
+[Package Managers](package_managers.md).
 
 ## Replace deprecated functions
 
@@ -9,7 +11,7 @@ deprecations are annotated with
 [`HEDLEY_DEPRECATED_FOR`](https://nemequ.github.io/hedley/api-reference.html#HEDLEY_DEPRECATED_FOR) to report which
 function to use instead.
 
-#### Parsing
+### Parsing
 
 - Function `friend std::istream& operator<<(basic_json&, std::istream&)` is deprecated since 3.0.0. Please use
   [`friend std::istream&  operator>>(std::istream&, basic_json&)`](../api/operator_gtgt.md) instead.
@@ -33,9 +35,11 @@ function to use instead.
 - Passing iterator pairs or pointer/length pairs to parsing functions ([`parse`](../api/basic_json/parse.md),
   [`accept`](../api/basic_json/accept.md), [`sax_parse`](../api/basic_json/sax_parse.md),
   [`from_cbor`](../api/basic_json/from_cbor.md), [`from_msgpack`](../api/basic_json/from_msgpack.md),
-  [`from_ubjson`](../api/basic_json/from_ubjson.md), and [`from_bson`](../api/basic_json/from_bson.md) via initializer
+  [`from_ubjson`](../api/basic_json/from_ubjson.md), and [`from_bson`](../api/basic_json/from_bson.md)) via initializer
   lists is deprecated since 3.8.0. Instead, pass two iterators; for instance, call `from_cbor(ptr, ptr+len)` instead of
-  `from_cbor({ptr, len})`.
+  `from_cbor({ptr, len})`. Likewise, passing a pointer and a length as two separate arguments to `from_cbor`,
+  `from_msgpack`, `from_ubjson`, and `from_bson` is deprecated since 3.8.0; call `from_cbor(ptr, ptr+len)` instead of
+  `from_cbor(ptr, len)`.
 
     === "Deprecated"
   
@@ -51,7 +55,7 @@ function to use instead.
           bool ok = nlohmann::json::accept(s, s + std::strlen(s));
           ```
 
-#### JSON Pointers
+### JSON Pointers
 
 - Comparing JSON Pointers with strings via [`operator==`](../api/json_pointer/operator_eq.md) and
   [`operator!=`](../api/json_pointer/operator_ne.md) is deprecated since 3.11.2. To compare a
@@ -93,7 +97,9 @@ function to use instead.
 
 - Passing a `basic_json` specialization as template parameter `RefStringType` to
   [`json_pointer`](../api/json_pointer/index.md) is deprecated since 3.11.0. The string type can now be directly
-  provided.
+  provided. This also applies to passing such a JSON pointer to [`at`](../api/basic_json/at.md),
+  [`contains`](../api/basic_json/contains.md), [`operator[]`](../api/basic_json/operator%5B%5D.md), and
+  [`value`](../api/basic_json/value.md).
   
     === "Deprecated"
   
@@ -108,10 +114,11 @@ function to use instead.
           nlohmann::json_pointer<my_string_type> ptr("/foo/bar/1");
           ```
   
-    Thereby, `nlohmann::my_json::json_pointer` is an alias for `nlohmann::json_pointer<my_string_type>` and is always an 
-    alias to the `json_pointer` with the appropriate string type for all specializations of `basic_json`.
+    Thereby, `my_json::json_pointer` is an alias for `nlohmann::json_pointer<my_string_type>`; in general,
+    `basic_json::json_pointer` is always an alias to the `json_pointer` with the appropriate string type for all
+    specializations of `basic_json`.
 
-#### Miscellaneous functions
+### Miscellaneous functions
 
 - The function `iterator_wrapper` is deprecated since 3.1.0. Please use the member function
   [`items`](../api/basic_json/items.md) instead.
@@ -260,7 +267,7 @@ exact version and configuration is relevant, use macro
       }
       ```
 
-## Do not use the `details` namespace
+## Do not use the `detail` namespace
 
-The `details` namespace is not part of the public API of the library and can change in any version without an
-announcement. Do not rely on any function or type in the `details` namespace.
+The `nlohmann::detail` namespace is not part of the public API of the library and can change in any version without
+an announcement. Do not rely on any function or type in the `detail` namespace.