doc/netlink/specs: Add spec for nlctrl netlink family

Add a spec for the nlctrl family.

Example usage:

./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/nlctrl.yaml \
    --do getfamily --json '{"family-name": "nlctrl"}'

./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/nlctrl.yaml \
    --dump getpolicy --json '{"family-name": "nlctrl"}'

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240306231046.97158-7-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>