Summary
This MR introduces a CONTRIBUTING.md to the repo as well as templates for issue and MR creation.
The automatic relabelling of issues has been prepared: Whenever an issue is referenced as to be closed in a MR, the label switches from Doing to In Review to ease the issue board management. Due to unknown reasons the GitLab GUI displays the newly labelled issues in the board list "Open" instead of "In Review" (although the relabelling works fine technically), so this step has been commented out for now. Further investigation is needed.