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/basic_json_view/is_number.md

@@ -0,0 +1,47 @@
+# <small>nlohmann::basic_json_view::</small>is_number
+
+```cpp
+bool is_number() const noexcept;
+```
+
+This function returns `#!cpp true` if and only if the value is a number, i.e. an integer, unsigned integer, or floating-point value. It is defined as `#!cpp is_number_integer() || is_number_float()`.
+
+## Return value
+
+`#!cpp true` if the type is a number, `#!cpp false` otherwise.
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Examples
+
+??? example
+
+    The example below classifies several parsed documents by the type of their root value, without materializing any
+    of them into a `BasicJsonType` value.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__type_predicates.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__type_predicates.output"
+    ```
+
+## See also
+
+- [is_number_integer](is_number_integer.md) - return whether the value is an integer or unsigned integer number
+- [is_number_unsigned](is_number_unsigned.md) - return whether the value is an unsigned integer number
+- [is_number_float](is_number_float.md) - return whether the value is a floating-point number
+- [`BasicJsonType::is_number`](../basic_json/is_number.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.