Documentation: xsk: correct the obsolete references and examples

The modified lines are mainly related to the following commits[1][2]
which remove those tests and examples. Since samples/bpf has been
deprecated, we can refer to more examples that are easily searched
in the various xdp-projects, like the following link:
https://github.com/xdp-project/bpf-examples/tree/main/AF_XDP-example



[1]
commit f3660063 ("libbpf: move xsk.{c,h} into selftests/bpf")
[2]
commit cfb5a2db ("bpf, samples: Remove AF_XDP samples")

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250708062907.11557-1-kerneljasonxing@gmail.com


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