We're already installing each reference into its own namespaced
directory, so we don't need to add further path elements.
Additionally, with the changes in:
https://gitlab.gnome.org/GNOME/devhelp/-/merge_requests/20
https://gitlab.gnome.org/GNOME/gi-docgen/-/merge_requests/54
we can browse the GTK API references in DevHelp.
gidocgen = find_program('gi-docgen', required: get_option('gtk_doc'))
-docs_dir = gtk_datadir / 'doc/gtk4/reference'
+docs_dir = gtk_datadir / 'doc'
subdir('gdk')
subdir('gsk')