Use Dependabot to monitor actions

[madelen-at-work]

Describe your changes

We can now use Dependabot to help keep track of, and suggest updates, to dependency versions.
This commit will enable scanning of the actions we use.

Issue ticket number and link

• Fixes How to handle third-party actions in workflows #39

Checklist before requesting a review

• I have performed a self-review of my own code
• I have verified that the code builds perfectly fine on my local system
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have verified that my code follows the style already available in the repository
• I have made corresponding changes to the documentation