Allowing `_doc` as a type will enable users to make the transition to 7.0
smoother since the index APIs will be `PUT index/_doc/id` and `POST index/_doc`.
This also moves most of the documentation to `_doc` as a type name.
Closes#27750Closes#27751
Currently global ordinals are documented under `fielddata`. It moves them to
their own file since they also work with doc values and fielddata is on the way
out.
Closes#23101