SK-Go optical endstop sensor mount

Goals for this mount:
- works in Y-rails position B (rear position)
- easy to print
- easy to mount
- minimum material use

Mounts in the Y max position (the screw is used to keep the endstop mount in place)

IMPORTANT: The homing sequence has to change from XYZ to YXZ and the homing direction of Y.

How this can be achieved and how the Y axis offset can be dealt with (temporarily extend the range of the axis), is described and the config I use is hosted here:

Homing config: https://github.com/sprhns/rrf-homing

Printed parts required:
- 1 x sensors_mount
- 1 x Y_trigger
- 1 x X_trigger_standalone **or** X_trigger_piggy_back (when the backplate is occupied with: https://www.thingiverse.com/thing:4700426)






Additional non-printable parts required:

- 1 x M3 x 14 mm screw or 1 x M3 x 16 mm screw with 2 x M3 0.9 washers (optional - only cosmetic)
- 3 x M3 x 10 mm screws
- 4 x M3 nuts
- 2 x Lerdge optical endstops