doc/netlink/specs: Add draft nftables spec

Add a spec for nftables that has nearly complete coverage of the ops,
but limited coverage of rule types and subexpressions.
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240418104737.77914-2-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>