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