ci: add dependabot config

[afuetterer]

Closes # (if applicable).

Changes proposed in this Pull Request

This PR adds a Dependabot configuration to enable monthly updates for GitHub actions used in your CI workflows.

Ref: PyPSA/powerplantmatching#174

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[lkstrp]

Great! If you are on it, feel free to add it also to the other repos, e.g. atlite, linopy