Documentation: Add link to the project files of the case

[TheRealKasumi]

@rgrae81 kindly provided us a link to the project files of the 3D Models. They can be found here. It would be good if we can add it to the documentation. Proposal: maybe around the section of ordering the PCB/3D print???

[JakobLichterfeld]

I would also add a link in the model folder