Try gitlab symlink

I is a little off-putting to see github in the URL bar of the browser. In this MR I renamed file github.rst → gitrepo.rst and created a symlink github.rst → gitrepo.rst so that all old bookmarks will still work. I have tested this approach locally and it seems to work. Let's see if this will work on the servers.

EDIT: in fact I might just as well try to make it repository-agnostic - I renamed it to gitrepo.rst (I used gitlab.rst on first attempt)