GitLab Pages only really handles a single branch for each project, so
forking is the best way to have the proposed pages be rendered and
published by Pages.

By setting `baseURL` from CI using `$CI_PAGES_URL` we can ensure that
links are set properly instead of always pointing at
https://www.apertis.org/ as that's quite confusing during reviews.

However, for the main project we don't want to use the default GitLab
pages address, but we want to have links using the official
https://www.apertis.org/ address: to the main project now sets
`$BASEURL` as a CI/CD variable to point at https://www.apertis.org/


while forked projects will use the default based on `$CI_PAGES_URL`.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>