Imporved the release process of training operator

[Veer0x1]

What this PR does / why we need it:

Update release workflow:

• After a v*.*.* tag is pushed, a workflow will be triggered to build and push docker images to Docker Hub.
• After a v*.*.* tag is pushed, a workflow will be triggered to create a draft release.
• Add docs for releasing training operator.

Which issue(s) this PR fixes

Fixes #2155

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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[Veer0x1]

@andreyvelich please review

[andreyvelich]

Thank you for doing this @Veer0x1!
Given that we already release Kubeflow Training v1.9.0, please can you help us to implement this process for Kubeflow Training V2 as we discussed on the latest Training WG call?
This will be very valuable for us.

Also, can we consolidate changes between this PR and #2362 by @LogicalGuy77 to implement release automation for Kubeflow Training V2 ?