Allow Changelog to use abitrary text instead of hardcoded categories

[Jonas-Sander]

Currently we expect (some of) the following fields in the document (newFeatures, improvements, fixes, version, releaseDate). Through them we generate the headings in the text and print the text of the category below them. This means that we always have to use these specific categories.
Instead we might just use one text property which we fill with markdown. This allows us to just change the text arbitrarily and not chain us to specific categories. We would probably still need version to order them in the correct way?