doc/mgr: improve nfs.rst

Note that NFS can be colocated from Umbrella instead of claiming it
isn't supported. Add a label in cephadm/service/nfs.rst for linking to
more info.
Call it NFS-Ganesha consistently.
Use the prompt directive consistently for CLI commands and improve markup.
Some minor capitalization, punctuation, etc. improvements.

Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>