Move Haddock's ghc-head branch & worflow to Gitlab

As discussed before, we should probably move the ghc-head branch of Haddock to Gitlab. It makes little sense to ask Haddock maintainers to review PRs adapting Haddock to code that isn't even yet merged into GHC.

Ideally only Marge bot would have commit rights on the ghc-head branch and it would merge commits as it does for GHC head.