docs: ioctl: add it to the uAPI guide

While 100% of its contents is userspace, let's keep the dir
at the same place, as this is a well-known location.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/index.rst

@@ -56,6 +56,7 @@ the kernel interface as seen by application developers.
    :maxdepth: 2
 
    userspace-api/index
+   ioctl/index
 
 
 Introduction to kernel development

Documentation/ioctl/index.rst

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

Documentation/ioctl/ioctl-number.rst

-:orphan:
-
 =============
 Ioctl Numbers
 =============