docs: netlink: basic introduction to Netlink

Provide a bit of a brain dump of netlink related information
as documentation. Hopefully this will be useful to people
trying to navigate implementing YAML based parsing in languages
we won't be able to help with.

I started writing this doc while trying to figure out what
it'd take to widen the applicability of YAML to good old rtnl,
but the doc grew beyond that as it usually happens.

In all honesty a lot of this information is new to me as I usually
follow the "copy an existing example, drink to forget" process
of writing netlink user space, so reviews will be much appreciated.
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20220819200221.422801-2-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/userspace-api/index.rst

@@ -26,6 +26,7 @@ place where this information is gathered.
    ioctl/index
    iommu
    media/index
+   netlink/index
    sysfs-platform_profile
    vduse
    futex2

Documentation/userspace-api/netlink/index.rst

+.. SPDX-License-Identifier: BSD-3-Clause
+
+================
+Netlink Handbook
+================
+
+Netlink documentation for users.
+
+.. toctree::
+   :maxdepth: 2
+
+   intro