Test print just_leg_test.stl missing

The page 0_printing.md mentions the just_leg_test.stl twice but there is no reference to it. Had to find a closed issue which had the link that goes to the old GitHub Repo - https://github.com/rwb27/openflexure_microscope/blob/master/stl/just_leg_test.stl

Happy to make a pull/merge request with a link to the GitHub page as a stop gap but it may be good to also

• add it as the first file to print in the list to test if a new user's printer can print the rest of the files
• add it in the source scad files
• add it in the zip download folder as well