Documentation: netlink: specs: tc: Add DualPI2 specification

Documentation: netlink: specs: tc: Add DualPI2 specification

Introduce the specification of tc qdisc DualPI2 stats and attributes,
which is the reference implementation of IETF RFC9332 DualQ Coupled AQM
(https://datatracker.ietf.org/doc/html/rfc9332) providing two different
queues: low latency queue (L-queue) and classic queue (C-queue).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Link: https://patch.msgid.link/20250722095915.24485-7-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>