Add monthly reminders for EP recurring "maintenance tasks"

Context

We have a few tasks that we should probably do at least monthly (check test selection gaps, check pipeline P80 durations), that are currently do based on the good will of somebody, or that we forget to do when other things are pressing.

For instance, we didn't notice that the pipeline duration doubled over the last 5 months.

Goal

Make those tasks visible to the entire team and people inside our EP slack channel

Proposal

I'd like to propose a semi-manual, boring solution to that problem, as the very first step to automating processes if needs be:

• Have a RUNBOOK entry for both those tasks
• Configure a slack reminder for those

Why not automate those alerts directly, instead of having to check every month?

1. Sisense is going away for Tableau soon, so I don't want to spend some time depending on a system that will go away at the end of the year. We have a related issue for automating the pipeline duration alerts in gitlab-org/gitlab#361665 (closed).
2. I like boring solutions, and the first iteration of a boring solution is definitely slack reminders. They're easy to make, they do the job, and they're in a place we all check daily.