gitlab-ci: Restrict GitLab pages job to the "main" branch

@bmilar asked me for this. With this changes the pipeline indicator won't appear for other branches (and consequentially merge-requests).

I think that's not really desirable because this prevents you from previewing the pages before publishing.

What I suggest instead is to build pages for branches in different paths (urls).

@bmilar let me know if you'd rather go with my suggestion, so that I can provide a merge-request with a proposal.