Accordion FAQ was just right for the task.
Initially I had a problem that turned out to be due to me not reading the manual properly. Fixed it with a hint I found in the extension forum, and a very quick response from the author.
Not really a FAQ-specific extension, but a general pourpose accordion that could be used for many other things as well.
Only thing missing is folding sub-categories. Can be fixed with a bit of css, but would be nice if it was right there, ideally as an option.