Add nbspinx for documentation

nbspinx is a really convenient tool to make tutorials in the documentation look nice! It converts notebooks into rendered pages. I already tried it here: https://irp.pages.gwdg.de/Hitchhikers_Guide_to_IRP/02images.html But as Tim and Leon pointed out, it is better to have documentation at one place, and not at multiple places, so I will ship essential examples from there to here and delete the Hitchhikers_Guide_to_IRP in the end.