docs: AF_XDP Device Plugin

Fixup the references to the AF_XDP Device Plugin in
the documentation (was referred to as CNI previously)
and document the single netdev limitation for deploying
an AF_XDP based DPDK pod. Also renames af_xdp_cni.rst to
af_xdp_dp.rst

Fixes: 7fc6ae5 ("net/af_xdp: support CNI Integration")
Cc: [email protected]

Signed-off-by: Maryam Tahhan <[email protected]>
Signed-off-by: 0-day Robot <[email protected]>

Author: maryamtahhan