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/is_number_integer.md

@@ -40,6 +40,7 @@ Constant.
 - [is_number()](is_number.md) check if the value is a number
 - [is_number_unsigned()](is_number_unsigned.md) check if the value is an unsigned integer number
 - [is_number_float()](is_number_float.md) check if the value is a floating-point number
+- [basic_json_view::is_number_integer](../basic_json_view/is_number_integer.md) - the same check on a zero-copy view
 
 ## Version history