Move worker-versioning.md back to docs folder (#6480)

## What changed?
<!-- Describe what has changed in this PR -->
Move versioning docs to the previous place as it is referenced in the
public docs.

We can move/delete this file once versioning goes to public preview.

## Why?
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## How did you test it?
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test? Checked in staging env? -->

## Potential risks
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change to production? -->

## Documentation
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stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->

## Is hotfix candidate?
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