docs: move sphinx back to main repo

This is essentially a revert of !66 (merged). Moving the rendering back allows us to render the website directly from the repo (which is nice for developing) and to run it in the CI (which would have prevented e.g. !182 (merged)).

To do this, we need to exclude the news, since we definitely don't want to have this here. Luckily in Python we can just remove the news/index line from index.md in 6 just lines. Running make -C docs html automatically regenerates index.md.

I also fixed some missing references and changed the references such they also work when looking at them in GitLab.

This will also help for adding satellite documentation (#81), since we can just copy them over in docs/conf.py.

Website changes are in https://gitlab.desy.de/constellation/constellation.pages.desy.de/-/merge_requests/9