Add GitHub Actions configuration to Dependabot

[EwoutH]

This PR adds 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 few PRs, since only major versions are specified (like v3), so only on a major release (like v4) it will update and open a PR. But it helps actively keep GitHub Actions workflows up to date and secure.

See Keeping your actions up to date with Dependabot.