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>

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

@@ -11,6 +11,7 @@ Device Mapper
     dm-clone
     dm-crypt
     dm-dust
+    dm-ebs
     dm-flakey
     dm-init
     dm-integrity