CI: Publish docs on GitLab Pages
Publish our html docs on GitLab Pages. This is how GTK, libhandy, and others have been publishing their nightly reference documentation.
There's some point of failure caught by https://gitlab.gnome.org/GNOME/gtk-doc/-/blob/master/gtkdoc/scan.py#L1042
Not sure if it's a failure on gtk-doc for not checking for None, or something gtksourceview is doing wrong that is messing with gtk-doc.
Edited by Christopher Davis