doc/netlink: Add spec for rt link messages

Add schema for rt link with support for newlink, dellink, getlink,
setlink and getstats.

A dummy link can be created like this:

sudo ./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/rt_link.yaml \
    --do newlink --create \
    --json '{"ifname": "dummy0", "linkinfo": {"kind": "dummy"}}'

For example, offload stats can be fetched like this:

./tools/net/ynl/cli.py \
    --spec Documentation/netlink/specs/rt_link.yaml \
    --dump getstats --json '{ "filter-mask": 8 }'

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-12-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>