docs: added use cases for using prometheus-operator

[googs1025]

What would you like to be added:
When I was adding this feature #614, I noticed that although JobSet integrates prometheus metrics by default (using ServiceMonitor), there is no good use case (neither in the kueue jobset project), so I will add a yaml file and examples that use prometheus-operator metrics to make it easy for users to get started.
Why is this needed:

• make it easy for users to get started to use metrics

[googs1025]

I plan to add a new prometheus-operator directory under example/

[googs1025]

/assign

[googs1025]

/kind docs

[k8s-ci-robot]

@googs1025: The label(s) kind/docs cannot be applied, because the repository doesn't have them.

In response to this:

/kind docs

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[googs1025]

/kind documentation