Setup `dependabot` to update GHA workflow file action dependencies

For example, [`main.yml`](https://github.com/graph-algorithms/planarity/blob/master/.github/workflows/main.yaml) uses `actions/checkout` (currently `actions/checkout@v6`) 

See [Automating Dependabot with GitHub Actions](https://docs.github.com/en/code-security/tutorials/secure-your-dependencies/automating-dependabot-with-github-actions)