Adopt the `changelog-workflow`

[JanNash]

As mentioned in the readme, we want to introduce a changelog file in our repositories to keep track of features/fixes in PRs and deploys.

The versioning mentioned there might be a separate issue/discussion for the resmio-repo for example, since we don't currently tag actual versions there afaik (do we?). In other repositories, we do have versioning, so they should be ready to adopt the changelog-workflow already.

Also, when adopting it, we might want to add a few small examples in the git-along readme of how it should be formatted and elaborate a bit more on possible problems/edge-cases of the workflow when they come up.

[JannKleen]

Also add this to PR Guidelines (when we have them)