Add Dependabot for GitHub Actions

[petergoldstein]

There is an outdated checkout action in the CI config, so after this is merged Dependabot should open a PR to update that action.

[adamstegman]

Is there some documentation for this? I don't think I understand what this is enabling.

[petergoldstein]

@adamstegman Here's GitHub's documentation - https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot

[adamstegman]

Awesome, thank you!