GTK documentation gtk-doc source has been removed

[andyholmes]

Summary

The documentation is currently build from gtk-doc sources at https://developer-old.gnome.org/ which has been retired. See GNOME Discourse: Retirement of developer-old.gnome.org

Fix

To keep the scraper working, it will have to be ported to scrape documentation from e.g. https://docs.gtk.org/gtk4 and https://docs.gtk.org/gtk3. These have a different format and layout, however.