doc: include 'fs new' stuff in cephfs quickstart

Not sure how 'quick' this really is now compared with
the full filesystem instructions, but let's not leave
it incomplete.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2829 from ceph/wip-doc-fs-quickstart

doc: include 'fs new' stuff in cephfs quickstart