Publish Qt5 frontend API documentation via GitLab pages.
This adds a CI job to run Doxygen and publish the Qt5 frontend API documentation using GitLab's Pages functionality.
The job itself seems to work and create the API documentation, but there deploy currently fails, probably because the FDo admins did not yet configure a domain that can be used by that functionality. Or we might need to talk to them about configuring one for Poppler, e.g. by redirecting
poppler.freedesktop.org/docs or something similar.