Where can I find a description of German Language SE's policy on admissible question types and topics, either as a stand-alone page, or as an FAQ somewhere?
What topics can I ask about here? from the help center gives you an overview of the most important cases and is intended to answer exactly this question for first-time users.
Apart from this, you should find every previous discussion on on-topicness should be tagged on-topic here on Meta (and many of them are also linked on the above help-center page). If you find some Meta question that is not tagged on-topic but should be, please flag it or mention it in chat.