https://gitlab.haskell.org/ghc/ghc/wikis/working-conventions/git/submodules does give nice information. It refers to https://gitlab.haskell.org/ghc/ghc/wikis/repositories to learn about specific repositories. But is that second page correct? It refers to e.g. GitHub as the upstream for Haddock. Will our new GitLab-based CI pull from GitHub's Haddock? And what if a contributor doesn't have commit access?

Maybe it's all there, but I don't quite see how a contributor can ensure that CI has access to patches on submodules.