Ohhh. While reading some other threads in the forum I stumbled upon an issue with a “Pipeline Editor“ which was released in version 13.8 this year. Turns out that the feature I am looking for was added to this editor just some days ago:
Version 13.9.0 (2021-02-22) „View an expanded version of the CI/CD configuration” → see Release Notes for a feature description and introduction video.
Open project → Module »CI / CD« → Submodule »Editor« → Tab »View merged YAML«
Awesome! 