Fix URLs on forked projects by setting baseURL

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.

Rendered here: https://em.pages.apertis.org/apertis-website/