docs: Update {index,cyclades,pithos}.rst for v0.16

Update index, cyclades and pithos general documentation to reflect
v0.16 changes, most notably:

 * Implementation of the Volume Service and Cinder API
 * Integration of Archipelago as the only Pithos backend
docs: Add storage unification document

Add a new storage unification document describing the relations
and workflows between Objects, Images, Snapshots and Volumes.

Also reference it from the index page.