Documentation: nfs-rdma: convert to ReST

Convert nfs-rdma to ReST and move it to admin-guide. Content
remais mostly untouched. Also, mark the doc as obsolete.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/9c88f184f9de2a3eb5181563e258559efc02f58a.1578697871.git.dwlsalmeida@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>