dm: add documentation for dm-vdo target

This adds the admin-guide documentation for dm-vdo.

vdo.rst is the guide to using dm-vdo. vdo-design is an overview of the
design of dm-vdo.
Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>