Document json_document and json_view

- API pages for basic_json_document and basic_json_view, one per member,
  and for the four aliases, each with an example
- features/json_view.md: the problem the view solves, ownership and
  lifetime, what matches basic_json::parse() and what differs, and when
  to choose json, ordered_json, SAX, or the view
- the examples show why one would use the view, not only how: borrowed
  vs. owned input, reading a few fields and materializing one subtree,
  reusing a document across many messages
- registered in the mkdocs navigation, llms.txt, the docset, the
  exceptions page (out_of_range.416), architecture.md, the integration
  page, and the README; the yyjson credit is added to the README and
  license.md

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

docs/mkdocs/docs/api/ordered_json_document.md

@@ -0,0 +1,38 @@
+# <small>nlohmann::</small>ordered_json_document
+
+<small>Defined in header `<nlohmann/json_view.hpp>`</small>
+
+```cpp
+using ordered_json_document = basic_json_document<ordered_json>;
+```
+
+This type is a [`basic_json_document`](basic_json_document/index.md) of the [`ordered_json`](ordered_json.md)
+specialization: [`materialize()`](basic_json_view/materialize.md) on one of its views preserves the insertion order
+of object keys, instead of sorting them like [`json_document`](json_document.md) does.
+
+## Examples
+
+??? example
+
+    The example below demonstrates how `ordered_json_document` preserves the insertion order of object keys when
+    materializing.
+
+    ```cpp
+    --8<-- "examples/ordered_json_document.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/ordered_json_document.output"
+    ```
+
+## See also
+
+- [ordered_json_view](ordered_json_view.md) - a view of a value of an `ordered_json_document`
+- [json_document](json_document.md) - the corresponding document for the default `json` specialization
+- [Object Order](../features/object_order.md)
+
+## Version history
+
+Since version 3.13.0.