Add release actions

[simonwacker]

Two GitHub Actions to create releases.

1. Draft a new release with a new version according to Semantic Versioning by running the GitHub action which, in particular, creates a new branch named release/v*.*.*, where *.*.* is the version, and a corresponding pull request.
2. Fetch the release branch by running git fetch and check it out by running git checkout release/v*.*.*, where *.*.* is the version.
3. Prepare the release, in particular, clean the change-log up and add new placeholders for the release to come after this one, add, commit, and push the changes.
4. Publish the new release by merging the release branch into main whereby a new pull request from main into develop is created that you need to merge to finish off.