[CI/CD] Fix Doxygen documentation

The goal of this merge request is to fix the Doxygen documentaiton compilation for the repository and to publish it through GitLab Pages.

To use GitLab Pages you just need to have a task in you pipeline with the name pages and an artifact that exposes your static site in the directory output. You also need to enable GitLab Pages on you server. To do so you need to add gitlab_pages['enable'] = true to your gitlab.rb file.