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/cmake.md

@@ -5,7 +5,8 @@
 You can use the `nlohmann_json::nlohmann_json` interface target in CMake. This target populates the appropriate usage
 requirements for [`INTERFACE_INCLUDE_DIRECTORIES`](https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_INCLUDE_DIRECTORIES.html)
 to point to the appropriate include directories and [`INTERFACE_COMPILE_FEATURES`](https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_COMPILE_FEATURES.html)
-for the necessary C++11 flags.
+for the necessary C++11 flags. Most [package managers](package_managers.md) that provide a CMake package configuration
+for this library expose this same target.
 
 ### External
 
@@ -138,7 +139,7 @@ Enable [extended diagnostic messages](../home/exceptions.md#extended-diagnostic-
 !!! warning "Does not apply to a pre-installed package"
 
     This option only takes effect when building nlohmann/json from source as part of your own
-    CMake project (e.g. via [`FetchContent`](#fetchcontent) or [`add_subdirectory`](#external)).
+    CMake project (e.g. via [`FetchContent`](#fetchcontent) or [`add_subdirectory`](#embedded)).
     It has **no effect** on a package that was already built and installed elsewhere (Homebrew,
     vcpkg, a system package, etc.) — the resulting compile definition is baked into the exported
     `nlohmann_jsonTargets.cmake` at install time, and `set(JSON_Diagnostics ON)` before
@@ -182,15 +183,22 @@ Skip expensive/slow test suites. This option is `OFF` by default. Depends on `JS
 ### `JSON_GlobalUDLs`
 
 Place user-defined string literals in the global namespace by defining the macro
-[`JSON_USE_GLOBAL_UDLS`](../api/macros/json_use_global_udls.md). This option is `OFF` by default.
+[`JSON_USE_GLOBAL_UDLS`](../api/macros/json_use_global_udls.md). This option is `ON` by default; see the
+[migration guide](migration_guide.md#import-namespace-literals-for-udls) for how to prepare code for the next major
+release, where the literals are removed from the global namespace.
 
 ### `JSON_ImplicitConversions`
 
-Enable implicit conversions by defining macro [`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md). This option is `ON` by default.
+Enable implicit conversions by defining macro
+[`JSON_USE_IMPLICIT_CONVERSIONS`](../api/macros/json_use_implicit_conversions.md). This option is `ON` by default; see
+the [migration guide](migration_guide.md#replace-implicit-conversions) for how to prepare code for the next major
+release, where implicit conversions are switched off by default.
 
 ### `JSON_Install`
 
-Install CMake targets during install step. This option is `ON` by default if the library's CMake project is the top project.
+Install CMake targets during install step. This option is `ON` by default if the library's CMake project is the top
+project. Installing also generates a [pkg-config](pkg-config.md) file for tools that rely on `pkg-config` instead of
+CMake.
 
 ### `JSON_LegacyDiscardedValueComparison`
 
@@ -209,6 +217,13 @@ Treat the library headers like system headers (i.e., adding `SYSTEM` to the [`ta
 Reject a `'\0'` (NUL) byte in the input instead of treating it as end of input, by defining the macro
 [`JSON_STRICT_NUL_HANDLING`](../api/macros/json_strict_nul_handling.md). This option is `OFF` by default.
 
+### `JSON_TestSimdutf`
+
+Build the unit tests against the [simdutf](https://github.com/simdutf/simdutf) UTF-8 validation backend by defining
+[`JSON_USE_SIMDUTF`](../api/macros/json_use_simdutf.md) for every test target. simdutf is fetched during configuration;
+its version is set by the cache variable `JSON_SIMDUTF_VERSION`. This option is `OFF` by default. Depends on
+`JSON_BuildTests`.
+
 ### `JSON_Valgrind`
 
 Execute the test suite with [Valgrind](https://valgrind.org). This option is `OFF` by default. Depends on `JSON_BuildTests`.