Documentation: nfsroot.txt: convert to ReST

Convert nfsroot.txt to RST and move it to admin-guide. Content remains
mostly the same.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/442d35917351f5260dd8ed7362e9b5f1264ef8ad.1578697871.git.dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/nfs/index.rst

@@ -6,3 +6,4 @@ NFS
     :maxdepth: 1
 
     nfs-client
+    nfsroot