Raspberry Arm Camera for Prusa MK3

Based the arm on https://www.thingiverse.com/thing:2802522 and the Cam housing on https://www.thingiverse.com/thing:1646474:

- modded the arm so it does not move at the motor side
- brought the cam closer to the bed
- brought the cam at same level as the nozzle
- the cam enclosure sits loose in the arm for easy removal e.g. of printer from enclosure (when Raspberry sits outside the enclosure)
- added an example picture of the camera image while printing (to show the viewing angle)