Fix documentation gaps found in a full GitHub Discussions review (#5264)

* 📝 Fix documentation gaps found in a full GitHub Discussions review

Reviewed all 1008 GitHub Discussions (2020-2026) for recurring questions
that better or more visible documentation would have avoided. Adds/expands
documentation for ~26 distinct gaps, including:

- New "Debugging" page collecting natvis, GDB pretty printer, LLDB status,
  and JSON_DIAGNOSTICS pointers (previously scattered/undiscoverable)
- Thread-safety and schema-validation FAQ entries
- StringType's char-based requirement (no wstring/u16string/u32string)
- Brace-initialization-yields-arrays warning directly on the constructor
  reference page (previously only in the FAQ, missed by users reading
  the constructor docs)
- std::any exclusion from get<T>(), with a manual-dispatch example
- Non-string-keyed std::map serializing as an array of pairs
- ordered_json compatibility with NLOHMANN_DEFINE_TYPE_* macros
  (already worked, was undocumented)
- std::array truncation on size-mismatched conversion (no exception)
- static_cast vs. get<std::optional<T>>() divergence
- Recipe for omitting a std::optional field instead of emitting null
- No built-in nesting-depth limit during parsing + a callback-based
  workaround recipe
- Recipe for streaming a large homogeneous array via parser callbacks
- operator>> stream-position semantics for concatenated JSON values
- JSON Pointer array-vs-object creation rule for non-existing paths
- CMake target name (nlohmann_json_modules) needed to link C++20 modules
- ESP-IDF/PlatformIO: no official package, link to a community fork
- get(key, default) as the Python dict.get() equivalent
- reserve() recipe for pre-allocating array capacity
- JSONC as an alias for the existing ignore_comments/ignore_trailing_commas
  combination (distinct from the unsupported JSON5)
- items() dereferenced-element type: decltype() idiom + detail-namespace
  stability caveat
- Various macro/type-conversion limitations (MSGPACK_DEFINE_ARRAY
  equivalent, char-array round-tripping, ADL serializer macro gap)

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

* 🎨 fix format

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

---------

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

docs/mkdocs/docs/api/macros/nlohmann_define_type_intrusive.md

@@ -62,6 +62,9 @@ See the examples below for the concrete generated code.
 
     - The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types
       with more than 63 member variables, you need to define the `to_json`/`from_json` functions manually.
+    - These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant
+      that serializes a struct's members positionally into a JSON array; for that, write `to_json`/`from_json` by
+      hand, building/reading a `json::array()` of the members in order.
 
 ## Examples
 

docs/mkdocs/docs/api/macros/nlohmann_define_type_non_intrusive.md

@@ -63,6 +63,9 @@ See the examples below for the concrete generated code.
 
     - The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types
       with more than 63 member variables, you need to define the `to_json`/`from_json` functions manually.
+    - These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant
+      that serializes a struct's members positionally into a JSON array; for that, write `to_json`/`from_json` by
+      hand, building/reading a `json::array()` of the members in order.
 
 ## Examples