docs: Fix links to gitlab-foss
@marcel.amirault: Had a few minutes to check and fix some links. For checked links see the issue. Related to gitlab-docs#440 (closed)
I guess I can continue soonest next week, and I don't want master and my fork to diverge too much (conflicts etc). But if you don't care or don't want to merge this small changes I can put a WIP:
here.