ci: Build docs artifacts for deployment to docs.gtk.org
The actual deployment will be done by https://gitlab.gnome.org/GNOME/gtk/-/blob/docs-gtk-org/; it pulls the most recent artifact zip from glib.git.
This ensures that only one project/job/branch has push access to the website.
Signed-off-by: Philip Withnall pwithnall@gnome.org
Helps: #3037 (closed)