Add debugging instructions for review-apps TLS
Context
Related to #387931
Postmortem item of gitlab-org/gitlab#387931 (comment 1236930915).
Goal
Explain what to do if a review-app doesn't have a valid certificate as a first step.
Related to #387931
Postmortem item of gitlab-org/gitlab#387931 (comment 1236930915).
Explain what to do if a review-app doesn't have a valid certificate as a first step.