Add CADs from separate repo as submodule

Submodule is referencing https://gitlab.fabcity.hamburg/hardware/olsk-3d-printer-desktop-freecad.git

Now after cloning this repo, the CAD directory is there, but empty. You must run two commands: git submodule init to initialize your local configuration file, and git submodule update to fetch all the data from that project and check out the appropriate commit listed in your superproject.