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

Convert pnfs-block-server.txt 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/c06903760e690c16d9df92f5e75f80381d6326d8.1578697871.git.dwlsalmeida@gmail.com


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