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/home/faq.md

@@ -129,6 +129,29 @@ As described [above](#parse-errors-reading-non-ascii-characters), the library as
     }
     ```
 
+## Usage
+
+### Thread safety
+
+!!! question
+
+    Is `basic_json` thread-safe?
+
+No. `basic_json` provides no built-in synchronization, the same as `std::map` or `std::vector`. Concurrent reads of
+the same value from multiple threads are safe, as are concurrent (non-overlapping) accesses to independent `json`
+objects. However, any concurrent write to a `json` object -- or a concurrent read while another thread writes to the
+same object -- is a data race and requires external synchronization (e.g., a `std::mutex`) by the caller.
+
+### Schema validation
+
+!!! question
+
+    Does this library support JSON Schema validation?
+
+Not directly, but the companion project [json-schema-validator](https://github.com/pboettch/json-schema-validator)
+builds JSON Schema (draft 4, 6, 7, and 2019-09) validation on top of this library and is a common recommendation
+for this use case.
+
 ## Exceptions
 
 ### Parsing without exceptions