Documentation: netlink: add a YAML spec for mptcp

it describes most of the current netlink interface (uAPI definitions,
doit/dumpit operations and attributes)

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/340


Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-1-v2-4-16b1f701f900@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>