Add docs/example of how to do self-hosted GH runners

[DominicOram]

As a user I would like an example of how to do self-hosted GH runners in my project e.g. see https://dev-portal.diamond.ac.uk/guide/kubernetes/tutorials/arc/.

Should we add a link to these docs (or the example from the docs) to this repo?

[coretl]

I think a generic how-to on using self-hosted GH runners is useful, then a link to those internal docs that will say which runners are available on DiamondLightSource