bpf, docs: DEVMAPs and XDP_REDIRECT

Add documentation for BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH
including kernel version introduced, usage and examples.

Add documentation that describes XDP_REDIRECT.

Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221115144921.165483-1-mtahhan@redhat.com