Adjust some aspects of the project

Just very minor tweaks to this project (following on from !7 (merged)). I thought it might be worth adding markdownlint configuration for the first time and make the project adhere to it.

The main change is to not ask for merge rights on gitlab-runner (many of us have it historically) at all, but just rely on access given using the maintainer group. I think @sselhorn and @marcel.amirault could validate that.

Apart from that, just a few misc changes.

@aqualls did you want to review this as the last person to modify the project in this space?