doc: update vpp-l3fwd user guide

- Remove unused section
- Add steps on setup and build
- Validate ip route entry
- Add extra port guide
- Add links to support other types of NIC
- Add advanced instructions

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

Change-Id: Ieb4ae759aa6dcaf1d6b6b2ee9a27862a3719bacb