Update presubmit and post submit job triggers

[Jeffwan]

Presubmit

I notice /manifests change doesn't trigger CI pipeline which doesn't give us confidence on correctness.
https://github.com/kubeflow/tf-operator/blob/e78151fb989b219f516ce77d22f1bdbfb4821215/prow_config.yaml#L12-L27

We should make the changes to includes the manifest folder. This has not been updated for long time. It's better to revisit the settings and includes folder required CI pipeline.

Postsubmit

There're two options.

1. trigger post submit job for every commit.
   Every time we release the image, we can pick up latest one.

https://github.com/kubeflow/tf-operator/blob/e78151fb989b219f516ce77d22f1bdbfb4821215/prow_config.yaml#L38-L53

2. write a separate release workflow to build public images and release team could manually trigger the workflow.

[stale]

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