docs: infiniband: add it to the driver-api bookset

While this contains some uAPI stuff, it was intended to be read by a
kernel doc. So, let's not move it to a different dir, but, instead, just
add it to the driver-api bookset.
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>

Documentation/index.rst

@@ -90,6 +90,7 @@ needed).
 
    driver-api/index
    core-api/index
+   infiniband/index
    media/index
    networking/index
    input/index

Documentation/infiniband/index.rst

-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ==========
 InfiniBand