Added a 3D Model of a matrix display stand to the project

[davepl]

I added the 3D Model I've been using to print out matrix display stands to the project. Sadly, I can only find the GCODE and not the STL file, but hopefully others can import the GCODE to their favorite app/slicer/etc.

If the GCODE is of no use to anyone, let me know, and I'll dig more for the STL file or try to recreate it.