docs/usecase: add vpp L2 switching usecase

- Add vpp L2 switching case setup scripts
- Add vpp L2 switching case guide
- Support for DPDK and memif interface

Signed-off-by: Yefeng Lin <yefeng.lin@arm.com>
Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I24ddbee77541a508c6b2804e1308aa6f59235a46