[Chore][Samples] Rename ray-cluster.mini.yaml and add workerGroupSpecs

[MortalHappiness]

Why are these changes needed?

There are ray-job.sample.yaml and ray-service.sample.yaml but no ray-cluster.sample.yaml. Also it seems that ray-cluster.mini.yaml is not useful because it does not specify workerGroupSpecs. Therefore, this PR:

1. Rename ray-cluster.mini.yaml to ray-cluster.sample.yaml.
2. Add workerGroupSpecs section in that file.

Related issue number

Checks

• [x] I've made sure the tests are passing.
• Testing Strategy
  • [ ] Unit tests
  • [x] Manual tests
  • [ ] This PR is not tested :(