Review Apps for GitLab Pages

This came from the CI/CD Brainstorm at git-push-mexico.

We currently use Review Apps for the docs site, and it's absolutely amazing, but I have no idea how to set them up myself. I don't know how GitLab Pages works from a technical perspective, but it'd be amazing to have this automatically work with GitLab Pages. We already have the ability to deploy static sites to it, we just need to be able to deploy Review Apps to them. If it's not reasonable to use the branch name in the URL, maybe just use a randomly generated string as the name of the project and host it, e.g. at nasjkdasbjzcbzjxkhdasd.gitlab.io?

We are already doing this for www-gitlab-com, so we can learn from this example.

cc: @markpundsack @ayufan