Update workflow and docs for releasing Training Operator

[LogicalGuy77]

What this PR does / why we need it:

1. The release workflow is triggered when VERSION file is updated with new version in vX.Y-branch
2. The release workflow will:

• Validate version consistency
• Build and publish SDK to PyPI
• Build and push container images
• Add semantic versioning image tags every for the release as per
• Create Git tag
• Generate changelog
• Create draft GitHub release

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
Fixes #2155

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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