Add support for the redirect facility named gitlab

GitLab Pages now supports redirects. See https://docs.gitlab.com/ee/user/project/pages/redirects.html. The facility is almost identical to the one supported by Netlify. In fact, using the netlify redirect facility in Antora may already work. If it does already work, Antora should honor gitlab as an alias to netlify. If it doesn't work, Antora should offer one named gitlab that does (though the logic is likely to be very similar). The documentation should also be updated accordingly.