Document steps for creating Kubernetes credentials

The Kubernetes credentials used by `kubectl` and `kernelci.runtime.kubernetes` are currently created manually and imported as-is with a Docker volume. We'll need a way to reproduce them using a particular user account and permissions that don't give access to more resources than needed.

Start by tracing how the existing Kubernetes credentials are created manually and imported through the Docker volume for kubectl and kernelci.runtime.kubernetes. Document reproducible steps for a specified user account with only the required permissions; done means another operator can recreate and use the credentials without granting broader access.

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