dm vdo: add vdo documentation to device-mapper index

Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Documentation/admin-guide/device-mapper/index.rst

@@ -34,6 +34,8 @@ Device Mapper
     switch
     thin-provisioning
     unstriped
+    vdo-design
+    vdo
     verity
     writecache
     zero