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/Makefile

@@ -1,3 +1,8 @@
+# The MkDocs 2.0 banners of Material for MkDocs and ProperDocs (via mkdocs-redirects) are printed to stderr, not
+# logged, so they do not affect --strict; silence them anyway.
+export NO_MKDOCS_2_WARNING := true
+export DISABLE_MKDOCS_2_WARNING := true
+
 # serve the site locally
 serve: style_check
 	venv/bin/mkdocs serve
@@ -8,11 +13,17 @@ serve_dirty: style_check
 # This target is used in the CI (ci_test_build_documentation).
 # This target is used by the docset Makefile.
 build: style_check
-	venv/bin/mkdocs build
+	venv/bin/mkdocs build --strict
 
 style_check:
 	@cd docs ; ../venv/bin/python3 ../scripts/check_structure.py
 
+# check that all Mermaid diagrams parse (needs Node.js)
+# This target is used in the CI (ci_test_documentation_mermaid).
+check_mermaid:
+	npm ci --prefix scripts/mermaid --ignore-scripts --no-audit --no-fund
+	node scripts/mermaid/check_mermaid.mjs docs
+
 # check the links in the documentation files in docs/mkdocs
 link_check:
 	ENABLED_HTMLPROOFER=true venv/bin/mkdocs build