Create a workflow for certificate renewing mechanism

[danopt]

We should create a GH Action workflow for the certificate renewing mechanism. This will make debugging much easier.

[widhalmt]

You mean to test the workflow. Be my guest, that sounds like a good. I'm just not sure how that could work. Hm... on the other hand... we could create certificates in prepare.yml and refresh them in converge.yml. Would that be feasible?