Add Dependabot configuration for GitHub Actions updates
Add a Dependabot configuration that checks once a week if the GitHub Actions are still using the latest version. If not, it opens a PR to update them.

It will actually open very few PRs, since we only have major versions specified (like v3), so only on a major v4 release it will update and open a PR.

See [Keeping your actions up to date with Dependabot](https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot).
Thanks for the pull request, but I also think this needs some discussion in an issue first to see whether the maintainers want this and which dependency updater we want (for example there's also renovate which Ibis uses: https://github.com/marketplace/renovate)

Closing for now, but happy to have an issue to discuss