Add QFN-56 0.5mm EP 6.1mm

This is a dimension used by RHD2000 Series Digital Electrophysiology Interface Chips.

Previous MRs:

Relates to: kicad-symbols!878 (closed) Relates to: https://github.com/KiCad/kicad-footprints/pull/1294

Current MRs:

Footprint generator: kicad-footprint-generator!892 (merged)

The diagram doesn't show the tolerances: