doc/netlink: Add spec for rt addr messages

Add schema for rt addr with support for:
     - newaddr, deladdr, getaddr (dump)

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20230825122756.7603-11-donald.hunter@gmail.com


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