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/empty.md

@@ -0,0 +1,61 @@
+# <small>nlohmann::basic_json_view::</small>empty
+
+```cpp
+bool empty() const noexcept;
+```
+
+Checks whether [`size()`](size.md) is `0`, as [`BasicJsonType::empty()`](../basic_json/empty.md) would for the same
+value.
+
+## Return value
+
+The return value depends on the type and is defined as follows:
+
+| Value type          | return value    |
+|----------------------|-----------------|
+| null                  | `#!cpp true`     |
+| discarded             | `#!cpp true`     |
+| boolean               | `#!cpp false`    |
+| string                | `#!cpp false`    |
+| number                | `#!cpp false`    |
+| object                | `#!cpp object_t::empty()` |
+| array                 | `#!cpp array_t::empty()`  |
+
+## Exception safety
+
+No-throw guarantee: this function never throws exceptions.
+
+## Complexity
+
+Constant.
+
+## Notes
+
+As for [`BasicJsonType::empty()`](../basic_json/empty.md), this does not return whether a string value is empty -- it
+is `#!cpp false` for any string, regardless of its length.
+
+## Examples
+
+??? example
+
+    The example below uses [`size()`](size.md) and `empty()` to decide whether a parsed message is worth acting on,
+    without materializing it into a `BasicJsonType` value.
+
+    ```cpp
+    --8<-- "examples/basic_json_view__size_empty.cpp"
+    ```
+
+    Output:
+
+    ```json
+    --8<-- "examples/basic_json_view__size_empty.output"
+    ```
+
+## See also
+
+- [size](size.md) - return the number of elements
+- [`BasicJsonType::empty`](../basic_json/empty.md) - the corresponding function of `basic_json`
+
+## Version history
+
+- Added in version 3.13.0.