[Maturity model] Add a reason when skipping a criterion

In &587, we are looking forward to uplift the maturity levels of all services to level 1 (from level 0). Some criteria are irrelevant for a number of services. In #826 (closed), we made it possible to skip a criterion. When looking back the skip list, people usually forget the reason why a service criterion is skipped. We have too many services for all an individual brain to remember. Other people may raise this question again. So, I think it makes sense to add a reason whenever a criterion is skipped. In addition to the manifest, we will have to update how the handbook page is rendered as well.

• Tweak the runbook to add a slot for reason in the service manifest
• Update the handbook to display reasons
• Fulfill the reasons for existing skipped criteria