📡 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>

docs/mkdocs/docs/api/basic_json/operator[].md

@@ -124,6 +124,15 @@ Strong exception safety: if an exception occurs, the original value stays intact
       filled with `#!json null`.
     - The special value `-` is treated as a synonym for the index past the end.
 
+    !!! note "Creating intermediate levels that don't exist yet"
+
+        When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing
+        leaf), each missing level is created as an array or an object depending on whether the corresponding
+        pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token
+        creates an object. For example, on an initially `#!json null` value, `/foo/0/0/0` creates nested arrays,
+        while `/foo/one/one/one` creates nested objects. This is not specified by the JSON Pointer RFC; it is
+        this library's own, intentional disambiguation rule. See also [JSON Pointer](../../features/json_pointer.md).
+
 ## Examples
 
 ??? example "Example: (1) access specified array element"