Simplify instructions for debugging review apps

Where to see it?

Read the rendered markdown version.

Context

This is one of the personal action points I wrote as part of gitlab-org/gitlab#361833 (closed).

I thought it would be useful to have actionable instructions when seeing jobs in #review-apps-broken. I would like to follow this approach for several other processes we have (another example that comes to mind would be gitlab-org/quality/triage-reports#7547 (closed)).

What's in this MR?

• Add a #review-apps-broken Slack channel isn't empty section
• Regroup the previous problems related to a failing CI/CD review-app job under this section (it makes it clearer I think)
• Simplify the diagnosis part by using the GKE workloads (credits to @kwiebers, as I borrowed it from him).