docs: cleanup quickstart guide

This commit cleans up multiple things in the quickstart
guide. Notably, it:
    - applies consistent formatting
    - applies consistent naming
    - rearranges some headings and their levels
    - improves phrasing of certain sections for clarity and brevity
    - keeps setup.sh echos consistent with quickstart guide
    - has setup.sh install lshw package in case user does not have it
      installed

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

Change-Id: I951e94fae5d580d9b816321b6b809dc36e451ee9