doc: general improvements to quickstart guide

Add information on expected end users and their skills.
Improve DUT hardware and software requirements.
Add reference to changelog.

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: Iafe47541833bd12adedbbcfc2381fa64f311d900