Revert ROI_CROP_TOP to zero and make sure getting started addresses this

[Ezward]

The current default is ROI_CROP_TOP=45 which is generally ok for a standard 1/16 donkey car. However it is almost certainly wrong for any other car. Cropping is critical to success in the real world. I suggest that we revert this to zero and make sure we have good documentation for setting this as part of the getting start process; probably after calibration and before training.

NOTE that the ROI_* configurations are only applied if the string 'CROP' is added to the TRANSFORMATIONS array in the configuration file.

[Ezward]

This is not necessary because ROI_* configurations are only applied if 'CROP' is added to the TRANSFORMATIONS array, so no cropping is applied by default.