scsi: docs: Add an empty index file for SCSI documents

In preparation for adding the SCSI documents to the documentation
body, add an empty index for it.

The next patches should be adding contents to it, as files get
converted to ReST format.

Link: https://lore.kernel.org/r/4d8c1b7ebe5898ac4a8265ca5e5a9552da3b426f.1583136624.git.mchehab+huawei@kernel.orgSigned-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Documentation/index.rst

@@ -130,6 +130,7 @@ needed).
    bpf/index
    usb/index
    PCI/index
+   scsi/index
    misc-devices/index
    mic/index
    scheduler/index

Documentation/scsi/index.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+====================
+Linux SCSI Subsystem
+====================
+
+.. toctree::
+   :maxdepth: 1
+
+