Document external-gha-runner runtime configuration for CNCF projects

[RobertKielty]

CNCF Project GitHub Action developers need to know and understand what they're getting when they use one of the runners.

• Sample workflow usage
• Reference documentation for the runners we have defined in cluster/PROVIDER/runners

[RobertKielty]

Have the e2e tests generate release notes that runner users can look up to see what the are getting when they use a specific runner.

We can dig into e2e test actions to check how a runner looks at run time but ideally we want to be able to refer users on to generated docs.

cc @jeefy @koksay