Update docs/assignments/week18.md, docs/images/week18/cnc_bottom.jpg,...
Update docs/assignments/week18.md, docs/images/week18/cnc_bottom.jpg, docs/images/week18/cnc_top_1.jpg, docs/images/week18/cnc_top_2.jpg, docs/images/week18/cnc_top_3.jpg, docs/images/week18/cnc_top_4.jpg, docs/images/week18/top_cnc_v1.jpg, docs/images/week18/top_surfacing_cnc_v1.jpg, docs/images/week18/bottom_cnc_v1.jpg files