docs: describe vpp and how it works

Briefly decribe what VPP IPv4 L3fwd is and how it works in user guide.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I63928fe07db4b773b5249764bb034c95c8ebf576