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/index.md

@@ -0,0 +1,82 @@
+# <small>nlohmann::</small>basic_json_view
+
+<small>Defined in header `<nlohmann/json_view.hpp>`</small>
+
+```cpp
+template<typename BasicJsonType>
+class basic_json_view;
+```
+
+A read-only handle to one value of a [`basic_json_document`](../basic_json_document/index.md): two pointers (a pointer
+to the document and a pointer into its index), trivially copyable. A view is valid as long as
+
+- the document is alive,
+- the document has not been re-parsed with [`read()`](../basic_json_document/read.md) (or
+  [`parse()`](../basic_json_document/parse.md) into it) or shrunk with
+  [`shrink_to_fit()`](../basic_json_document/shrink_to_fit.md) since the view was taken, and
+- if the document borrows its source text, that text is still alive.
+
+Moving the document itself does not invalidate its views: the index is heap-allocated independently of the
+`basic_json_document` object.
+
+`basic_json_view` provides the read-only part of the `BasicJsonType` interface: the type-inspection functions, and
+[`materialize()`](materialize.md) to build the `BasicJsonType` value of a subtree on demand. It does not (yet) provide
+element access, iteration, `get<T>()`, JSON Pointer support, `dump()`, or comparison.
+
+## Template parameters
+
+`BasicJsonType`
+:   a specialization of [`basic_json`](../basic_json/index.md), matching the
+    [`basic_json_document`](../basic_json_document/index.md) the view was taken from.
+
+## Specializations
+
+- [**json_view**](../json_view.md) - views of a [`json_document`](../json_document.md)
+- [**ordered_json_view**](../ordered_json_view.md) - views of an [`ordered_json_document`](../ordered_json_document.md)
+
+## Member types
+
+- **value_t** - the JSON type enumeration, see [`basic_json::value_t`](../basic_json/value_t.md)
+- **string_t**, **number_integer_t**, **number_unsigned_t**, **number_float_t**, **json_pointer** - the corresponding
+  member types of `BasicJsonType`
+- **size_type** - `#!cpp std::size_t`
+- **string_view_t** - `#!cpp std::string_view` on C++17 and newer, a minimal internal substitute otherwise
+
+## Member functions
+
+- [(constructor)](basic_json_view.md)
+
+### Object inspection
+
+- [**type**](type.md) - return the type of the value
+- [**is_null**](is_null.md) - return whether the value is null
+- [**is_boolean**](is_boolean.md) - return whether the value is a boolean
+- [**is_number**](is_number.md) - return whether the value is a number
+- [**is_number_integer**](is_number_integer.md) - return whether the value is an 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
+- [**is_string**](is_string.md) - return whether the value is a string
+- [**is_array**](is_array.md) - return whether the value is an array
+- [**is_object**](is_object.md) - return whether the value is an object
+- [**is_binary**](is_binary.md) - return whether the value is a binary array (always `#!cpp false`)
+- [**is_primitive**](is_primitive.md) - return whether the type is primitive
+- [**is_structured**](is_structured.md) - return whether the type is structured
+- [**is_discarded**](is_discarded.md) - return whether the view is invalid
+- [**operator bool**](operator_bool.md) - return whether the view refers to a value
+
+### Capacity
+
+- [**size**](size.md) - return the number of elements
+- [**empty**](empty.md) - return whether the value has no elements
+
+### Conversion
+
+- [**materialize**](materialize.md) - build the `BasicJsonType` value of this subtree
+
+### Source access
+
+- [**source_offset**](source_offset.md) - byte offset of this value in the document's source text
+
+## Version history
+
+- Added in version 3.13.0.