doc/netlink/specs: Add a spec for tc

This is a work-in-progress spec for tc that covers:
 - most of the qdiscs
 - the flower classifier
 - new, del, get for qdisc, chain, class and filter

Notable omissions:
 - most of the stats attrs are left as binary blobs
 - notifications are not yet implemented

Signed-off-by: Donald Hunter <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: donaldh