doc: Minor improvements to Ceph FS landing page.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added another instance term to the glossary.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Added more glossary-compliant terms and indexing.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>