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.
 

docs/mkdocs/docs/api/basic_json/binary_t.md

@@ -37,6 +37,14 @@ represent a byte array in modern C++.
 `BinaryType`
 :   container type to store arrays
 
+    Although not formally expressed as a C++ concept, `BinaryType` must be default-constructible,
+    copy/move-constructible, and support `push_back()`, `.data()`, and `.size()`, because
+    [`byte_container_with_subtype`](../byte_container_with_subtype/index.md) derives directly from it. Its
+    `value_type` must additionally be exactly one byte wide (e.g., `std::uint8_t`/`char`/`std::byte`): the binary
+    serializers (CBOR, MessagePack, BSON, UBJSON) read and write the container's raw bytes via
+    `reinterpret_cast`, which is only correct for byte-sized elements -- a container like
+    `#!cpp std::vector<std::intptr_t>` will not work as `BinaryType`.
+
 ## Notes
 
 #### Default type

docs/mkdocs/docs/api/basic_json/items.md

@@ -46,6 +46,17 @@ for (auto& [key, val] : j_object.items())
 }
 ```
 
+If you need to name the type of the dereferenced element explicitly (e.g., to write a standalone function that
+takes it as a parameter, or to use `items()` with `std::for_each`), use `decltype`:
+
+```cpp
+using element_type = decltype(*j_object.items().begin());
+```
+
+The per-element type (`iteration_proxy_value`) lives in the library's internal `detail` namespace and is
+intentionally unspecified as a stable, named type -- `decltype` is the supported way to obtain it, but its exact
+name/definition may change between versions.
+
 ## Return value
 
 iteration proxy object wrapping the current value with an interface to use in range-based for loops

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"

docs/mkdocs/docs/api/basic_json/string_t.md

@@ -18,6 +18,11 @@ JSON class into byte-sized characters during deserialization.
 :   the container to store strings (e.g., `std::string`). Note this container is used for keys/names in objects, see
     [object_t](object_t.md).
 
+    `StringType` must have a `char`-compatible `value_type`: the library relies on UTF-8/`char`-based storage and
+    processing internally, so `std::wstring`, `std::u16string`, and `std::u32string` are **not** valid choices for
+    `StringType`. To work with wide-character data, convert it to/from UTF-8 at the boundary instead -- see the
+    FAQ's [wide string handling](../../home/faq.md#wide-string-handling) section for a conversion recipe.
+
 ## Notes
 
 #### Default type

docs/mkdocs/docs/api/basic_json/value.md

@@ -17,6 +17,8 @@ ValueType value(const json_pointer& ptr,
                 const ValueType& default_value) const;
 ```
 
+This is equivalent to Python's `dict.get(key, default)`.
+
 1. Returns either a copy of an object's element at the specified key `key` or a given default value if no element with
    key `key` exists.