Allow to specify the documentation type in the GIR file
Currently we have a mix of gtk-doc and gi-docgen in the .gir files, it might make sense to add a field in the .gir file to specify that the documentation is of gtk-doc format or gi-docgen format. In my use-case it would allow me to automatically generate the right documentation in Valadoc but I believe that it would make sense to directly have the documentation from the .gir file in Devhelp or GNOME Builder in the future.