doc: Add validation documentation

Add information on the validation for the stack integration.

Issue-Id: SCM-5624
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Qi Feng <qi.feng@arm.com>
Change-Id: Id7d5288acf7b41ff5bd07a33db4b005c00e9c18d