cifsd: add index.rst in cifs documentation

Since more than one file is in the cifs document directory,
This patch add an index.rst.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Documentation/filesystems/cifs/index.rst

+===============================
+CIFS
+===============================
+
+
+.. toctree::
+   :maxdepth: 1
+
+   cifsd
+   cifsroot

Documentation/filesystems/index.rst

@@ -72,8 +72,7 @@ Documentation for filesystem implementations.
    befs
    bfs
    btrfs
-   cifs/cifsd
-   cifs/cifsroot
+   cifs/index
    ceph
    coda
    configfs