Y axis align tool for fixed gantry printers

Inspired by 2.4's flying gantry, making gantry partially flying, using bed mesh as indication, manually tune Y axis into one plane.

I use standard voron heatset, modify if needed.

Loosen one rear end of Y axis, use a long M3 bolt to support and adjust the gantry.

Calibrate sequence: z—tilt, then make one mesh, compare z_offset value of two rear corners, if the flying one is smaller, which means the distance between bed and rail is larger（for example，0.1mm）, so unscrew the supporting M3 bolt 1/5 turn. After adjustment, z-tilt and bedmesh, check result, repeat if needed.