Add Original_Prusa_i3_MK3S_MK3_platform.stl

This is the latest mesh for Prusa i3 MK3/MK3s made by Prusa Research with minor modification to fit in Prusa folder under "add printer"
reference :
https://manual.prusa3d.com/Guide/How+to+import+profiles+to+Cura+4.x+(Windows+&+macOS)/1421#_ga=2.78043415.205833688.1568467545-859345073.1567270611