doc: update and improve "mount using kernel driver" doc

doc: update and improve "mount using kernel driver" doc

Move the examples of mount command for Ceph cluster with CephX enabled
to the top of page, since it is enabled by default, improve explanation
around Ceph with multiple FSs, get rid of hash symbols before every
command (without them it's clear that the text is command and with them
the reader cannot use the commands directly), link fstab page, add how
mount in general looks, add prerequisites required for kernel mounts and
expand explanation wherever possible.

Fixes: https://tracker.ceph.com/issues/42220
Signed-off-by: Rishabh Dave <ridave@redhat.com>