Pin compare Action examples to release tags (not @main)

[askmy-stack]

Summary

`docs/github-action.md` documents:

```yaml
uses: askmy-stack/tool-semantics/.github/actions/compare@main
```

Consumers should pin to a release tag (or commit SHA) for reproducible CI.

Acceptance criteria

• Docs show `@vX.Y.Z` (or `@v0`) once a release exists — coordinate with the first-release issue
• Note that `@main` tracks tip and may break
• Optional: add a short "Versioning the Action" subsection (tag the repo; composite actions resolve from that tag)
• Example consumer snippet updated consistently

Related

Pairs with first GitHub Release / PyPI publish.