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/basic_json/basic_json.md

@@ -115,7 +115,22 @@ basic_json(basic_json&& other) noexcept;
    
     Function [`array()`](array.md) and [`object()`](object.md) force array and object creation from initializer lists,
     respectively.
-        
+
+    !!! warning "Brace initialization yields arrays"
+
+        Because this constructor takes an `initializer_list_t`, brace-initializing a `json`/`ordered_json` from
+        another `json` value wraps it in a single-element array rather than copying it:
+
+        ```cpp
+        json j1 = "hello";
+        json j2{j1};   // [!] j2 is ["hello"], NOT a copy of j1
+        json j3(j1);   // j3 is "hello" -- parentheses copy as expected
+        ```
+
+        See the FAQ entry on [brace initialization](../../home/faq.md#brace-initialization-yields-arrays) for the
+        full explanation, an opt-in macro to change this behavior, and how to explicitly create a single-element
+        array (`json::array({value})`) if that is what you want.
+
 6. Constructs a JSON array value by creating `cnt` copies of a passed value. In case `cnt` is `0`, an empty array is
    created.