Annotate each piece of retractable.scad

No changes to dimensions.
retractable.scad: Minor refactor to clarify tunnel size changes to follow
retractable.scad: use spacings that better fit a 2mm allen wrench
Merge branch 'hotend_probe' of https://github.com/brandonheller/kossel into hotend_probe
rounded hinge on probe_top, added flat spot and holes for microswitch in groove_mount, will still need to add pad or spacer in effector_open to trigger switch