Documentation review (#5257)

* 📝 Fix documentation gaps for 3.13.0 release (todos 138-142)

- Todo 138: Add "Known issues" section to modules.md with compiler-specific troubleshooting (GCC redefinition, MSVC symbol export). Add pointer note to quality_assurance.md.
- Todo 139: Document CBOR/MessagePack half-precision float encoding for NaN/Infinity (0xF9/0xCA with exact byte sequences). Explain pre-3.13.0 double-precision bug mechanism without issue citations.
- Todo 140: Document CBOR negative-integer-overflow rejection (parse_error.112) for magnitudes exceeding int64_t range (already implemented in rev 1).
- Todo 141: Update version history in value.md and operator[].md with behavior-change details, removing issue citations per citation policy (prose is self-contained).
- Todo 142: Global sed replace of 3.12.x → 3.13.0 placeholder across all 20 documentation files.

Revision 2 incorporates feedback to reduce changelog-like issue citations. Only citations that add unique troubleshooting value are retained (#5103 for GCC workaround, #3970 for MSVC symbol export). "Known issues" section follows PR #5252's visual pattern (info admonition with bold-bullet format).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* 📝 Document integer type selection, type_name() invalid value, and std::optional get() fix

- number_handling.md: clarify that positive/negative integers select
  unsigned/signed storage based on the leading minus sign (todo 143).
- type_name.md: document the new "invalid" return value for corrupted
  JSON values (todo 145).
- get.md: note that get<std::optional<T>>() was unreachable in every
  configuration prior to 3.13.0 due to an internal macro-guard bug,
  unrelated to JSON_USE_IMPLICIT_CONVERSIONS's actual effect (todo 144).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/home/faq.md

@@ -178,7 +178,7 @@ See [this section](../features/types/number_handling.md#number-serialization) on
     - Can I use `std::format("{}", j)` on a JSON value?
     - Can I use `fmt::format("{}", j)` or `fmt::print("{}", j)` (the [{fmt}](https://github.com/fmtlib/fmt) library) on a JSON value?
 
-`std::format` works out of the box since version 3.12.x, as long as the standard library provides
+`std::format` works out of the box since version 3.13.0, as long as the standard library provides
 `<format>` (see [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)); see
 [`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) for details, including the `#!cpp "{:#}"`
 pretty-print spec, indent widths (`#!cpp "{:2}"`), and custom indent characters (`#!cpp "{:.>#}"`).