doc: storage: clean up driver overview page
doc: storage: clean up driver overview page

Only moving content around and whitespace changes in this
commit. No content changes.

Signed-off-by: Ruth Fuchss <90c286bb8b7c8291b9f5a2a1d38dc233728381fe@canonical.com>

stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,lxc/lxd,lxc/lxd