Add Dependabot configuration for GitHub Actions updates

[EwoutH]

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 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. This way it helps actively keep GitHub Actions workflows up to date and secure, while minimizing maintenance burden.

See Keeping your actions up to date with Dependabot.

[EwoutH]

@maartenbreddels this is a crucial piece of infrastructure to keep this repo up to date. Could you review it, and if approved, merge it?