docs: Add DPDK devbind.py section to quickstart

Adds explanation of dpdk-devbind.py to quickstart guide.
Also fixes minor formatting issue in quickstart guide.

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

Change-Id: I1a8b96f791367ccbdfb0c0146e242b06aa9f8516