Statically visualise our workflow hierarchy

[Johennes]

Our GHA setup is quite sophisticated with workflows being kicked off by various triggers including across repositories. To better document and reason about the system, it would be helpful if we had a static visualization of what kicks off what. This might be possible to automate by programmatically parsing the on fields in all workflow files. For outputting we may use something like Mermaid or graphviz which could then be integrated into our existing docs.