add maintainer tasks

@gitlab-data/permifrost_maintainers

In order to make sure we're all able to do our part as maintainers I created this first iteration on a checklist that should be completed at a regular interval (I suggest at least weekly)

We can also discuss if we want to schedule a rotation for this.

I also updated our labeling, which is linked in the issue template here, so that we're using scoped labels to organize into a workflow/issue progress. We might need to update the roadmap board to match as well.