Add changelog building step to release workflow

[timobrembeck]

Motivation

With our growing number of contributors, resolving conflicts in the changelog file becomes more and more effort.

Proposed Solution

Add changelog entries as separate files first and assemble them later in the release workflow, similar to how the app team is doing it

Alternatives

• Add a specific line to a commit message body, e.g. [changelog], and then write a script which automatically uses the commit message subject as changelog entry of the issue which is fixed by that commit
• Add a section to PR descriptions which can be parsed by a script and assembled into a changelog file
• Completely automate the changelog and generate one entry for each fixed issue since the last release

Additional Context