Import all the xml documentation for types from https://github.com/xamarin/apple-api-docs.
Some of this documentation should probably be rewritten, and potentially moved
to conceptual documentation, in particular those that contain images (because
images can't be imported into xml documentation).
Note that the documentation hasn't been modified in any way; that's not the purpose of this PR. If documentation should be modified for whatever reason, it can be done in a later PR.
The xml documentation for members will come in a later PR.
Partial fix for https://github.com/xamarin/xamarin-macios/issues/17399.
7670afb6aa