doc: improve storage drivers reference

Separate features comparison table into local and non-local tables and
move them closer to the top. Expand features reference section with
details about each feature, and add more information about remote versus
shared storage and the benefit of remote storage over local storage for
instance migration. Remove optimized image transfer feature from tables
since it duplicates optimized volume transfer.

Signed-off-by: Minae Lee <minae.lee@canonical.com>

Author: minaelee