The previous calc was wrong. Between N points there are only N-1 gaps,
not N.

So changing AUTO_BED_LEVELING_GRID_POINTS to
(AUTO_BED_LEVELING_GRID_POINTS-1)

Documentation restructuring

Conflicts:
	README.md

Documentation directory

(For the SLOW_PWM_HEATERS option)

Fix grammar/typos in new Configuration.h option

Revert "issue 968 for dev branch"

(For the SLOW_PWM_HEATERS option)

moved/renamed documentation stuff to top-level directory

406*406 pixels

Update Marlin Firmware URL