Documentation: nfs: pnfs-scsi-server: convert to ReST

Convert pnfs-scsi-server to ReST and move it to admin-guide. Content
remains mostly unchanged.

Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/5c4b8af41ca0a427a3987535815bccf47a65d320.1578697871.git.dwlsalmeida@gmail.com


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