DOCS: Add some small piece about setting up a Gitlab-runner

This is based on me setting one up, but I kept it general with mostly linking to other documentation.
The idea is that to just provide some Pleroma-specific info, maybe give some pointers, and point to the right (external) docs.

docs/development/setting_up_a_gitlab_runner.md

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+# Setting up a Gitlab-runner
+
+When you push changes, a pipeline will start some automated jobs. These are done with so called [runners](https://docs.gitlab.com/runner/), services that run somewhere on a server and run these automated jobs. These jobs typically run tests and should pass. If not, you probably need to fix something.
+
+Generally, Pleroma provides a runner, so you don't need to set up your own. However, if for whatever reason you want to set up your own, here's some high level instructions.
+
+1. We use docker to run the jobs, so you should install that. For Debian, you need to allow non-free packages in the [source list](https://wiki.debian.org/SourcesList). Then you can install docker with `apt install docker-compose`.
+2. You can [install](https://docs.gitlab.com/runner/install/index.html) and [configure](https://docs.gitlab.com/runner/register/index.html) a Gitlab-runner. It's probably easiest to install from the packages, but there are other options as well.
+3. When registering the runner, you'll need some values. You can find them in the project under your own name. Choose "Settings", "CI/CD", and then expand "Runners". For executor you can choose "docker". For default image, you can use the image used in <https://git.pleroma.social/pleroma/pleroma/-/blob/develop/.gitlab-ci.yml#L1> (although it shouldn't matter much).