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/json_sax/binary.md

@@ -35,6 +35,12 @@ It is safe to move the passed binary value.
     --8<-- "examples/sax_parse__binary.output"
     ```
 
+## See also
+
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+- [SAX Interface](../../features/parsing/sax_interface.md) - the SAX interface article
+- [Binary Values](../../features/binary_values.md) - the article on binary values
+
 ## Version history
 
 - Added in version 3.8.0.

docs/mkdocs/docs/api/json_sax/boolean.md

@@ -31,6 +31,11 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+- [SAX Interface](../../features/parsing/sax_interface.md) - the SAX interface article
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/end_array.md

@@ -26,6 +26,11 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [start_array](start_array.md) - the beginning of an array was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/end_object.md

@@ -26,6 +26,12 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [start_object](start_object.md) - the beginning of an object was read
+- [key](key.md) - an object key was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/index.md

@@ -9,6 +9,27 @@ This class describes the SAX interface used by [sax_parse](../basic_json/sax_par
 different situations while the input is parsed. The boolean return value informs the parser whether to continue
 processing the input.
 
+For instance, parsing the JSON text `{"a": [1, true]}` triggers the following callbacks, in order:
+
+```mermaid
+sequenceDiagram
+    participant P as Parser
+    participant H as SAX handler
+
+    P->>H: start_object(elements)
+    P->>H: key("a")
+    P->>H: start_array(elements)
+    P->>H: number_unsigned(1)
+    P->>H: boolean(true)
+    P->>H: end_array()
+    P->>H: end_object()
+```
+
+Note `elements` is passed as `#!cpp std::numeric_limits<std::size_t>::max()` (i.e., "unknown") for JSON text input;
+only binary formats such as CBOR or MessagePack may report the actual number of elements in `start_object`/
+`start_array`. Also note that `1` is reported via `number_unsigned` rather than `number_integer` because it has no
+leading `-` sign.
+
 ## Template parameters
 
 `BasicJsonType`

docs/mkdocs/docs/api/json_sax/key.md

@@ -35,6 +35,12 @@ It is safe to move the passed object key value.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [start_object](start_object.md) - the beginning of an object was read
+- [end_object](end_object.md) - the end of an object was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/null.md

@@ -26,6 +26,11 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+- [SAX Interface](../../features/parsing/sax_interface.md) - the SAX interface article
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/number_float.md

@@ -34,6 +34,12 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [number_integer](number_integer.md) - an integer number was read
+- [number_unsigned](number_unsigned.md) - an unsigned integer number was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/number_integer.md

@@ -31,6 +31,12 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [number_unsigned](number_unsigned.md) - an unsigned integer number was read
+- [number_float](number_float.md) - a floating-point number was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/number_unsigned.md

@@ -31,6 +31,12 @@ Whether parsing should proceed.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [number_integer](number_integer.md) - an integer number was read
+- [number_float](number_float.md) - a floating-point number was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/parse_error.md

@@ -39,6 +39,12 @@ Whether parsing should proceed (**must return `#!cpp false`**).
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+- [Parsing and Exceptions](../../features/parsing/parse_exceptions.md) - the article on handling parse errors without
+  exceptions
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/start_array.md

@@ -35,6 +35,11 @@ Binary formats may report the number of elements.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [end_array](end_array.md) - the end of an array was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/start_object.md

@@ -35,6 +35,12 @@ Binary formats may report the number of elements.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [end_object](end_object.md) - the end of an object was read
+- [key](key.md) - an object key was read
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+
 ## Version history
 
 - Added in version 3.2.0.

docs/mkdocs/docs/api/json_sax/string.md

@@ -35,6 +35,11 @@ It is safe to move the passed string value.
     --8<-- "examples/sax_parse.output"
     ```
 
+## See also
+
+- [sax_parse](../basic_json/sax_parse.md) - SAX parser
+- [SAX Interface](../../features/parsing/sax_interface.md) - the SAX interface article
+
 ## Version history
 
 - Added in version 3.2.0.