doc: Update the Validate section in User Manual/Reproduce

1. Add description to validate Zephyr and output example
2. Rearrange the steps to validate Linux and add output example

Issue-Id: SCM-4371
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I6a6d14493072c3c0f388bbde3286db0861b9bf8f