Hi @NRottmann (& @dagar),
Awesome. The docs should certainly be abstracted and refer only to official repo's.
The existing doc on Gazebo we have on gazebo is here. The best thing would be to merge your instructions into those (this may be tough given that they are structured differently). Can you have a first look at how this might be best achieved. If you find that hard let me know and I'll take a shot at it (very happy to do so).
Ideally the end result would be generic instructions, with a section on running the simulator for your sub much like the other ones here.
NOTE: On quick scan my take on the current PX4 docs is that they are very "thin and linky" meaning that there is the assumption you will be happy to go to linked docs and find required dependencies etc. While this approach has the benefit that you link to official docs that should be kept up to date, in this case it looks like the linked docs make a lot of assumptions about your existing knowledge of gazebo packaging/dependencies. Upshot, I'd be more than happy to extend our docs with your approach explicitly listing out dependencies and how things are installed (unless there is strong disagreement from others)
Let me know how/when I can help. I'm at hamishwillee at gmail dot com if you want to discuss offline.