doc/netlink/specs: add netkit support to rt_link.yaml

Add netkit support to rt_link.yaml. Only forward(PASS) and
blackhole(DROP) policies are allowed to be set by user-space so I've
added only them to the yaml to avoid confusion.

Example:
  $ ./tools/net/ynl/cli.py \
     --spec Documentation/netlink/specs/rt_link.yaml \
     --do getlink --json '{"ifname": "netkit0"}' --output-json | jq
  ...
  "linkinfo": {
    "kind": "netkit",
    "data": {
      "primary": 1,
      "policy": "blackhole",
      "mode": "l2",
      "peer-policy": "forward"
    }
  },
  ...

Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20240806104531.3296718-1-razor@blackwall.org


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