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

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?

Review the repository's existing documentation structure and the linked Diamond Kubernetes ARC tutorial first. Document how to use self-hosted GitHub runners, or add the appropriate tutorial link, with the finished documentation making the setup path clear to project users.

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