docs: device-mapper: add dm-ebs.rst to an index file

Solves this Sphinx warning:
	Documentation/admin-guide/device-mapper/dm-ebs.rst: WARNING: document isn't included in any toctree

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>