[docs] Let gtk-doc build the correct index
[docs] Let gtk-doc build the correct index

Use the --name-space command line argument for gtk-doc to detect the
right namespace of JSON-GLib, and create a useful index.

GNOME/json-glib,Distrotech/json-glib,ebassi/json-glib,oerdnj/json-glib,robtaylor/json-glib-gvariant,brauliobo/json-glib,frida/json-glib,GNOME/json-glib,robtaylor/json-glib-gvariant,ebassi/json-glib,oerdnj/json-glib,brauliobo/json-glib,brauliobo/json-glib,oerdnj/json-glib,Distrotech/json-glib,Distrotech/json-glib,frida/json-glib,oerdnj/json-glib,ebassi/json-glib