Ender 3 HeroMe Auto-bed leveling

This is just a simple remix with minor enhancements. I found this method to be the most reliable and yet cheapest solution for auto-bed leveling in the Ender-3.

The M48 command on Marlin (probe repeat-ability test) gives the following results:
Mean: -0.009839 Min: -0.011 Max: -0.009 Range: 0.002
Standard Deviation: 0.000756
(this is on a stock ender-3 hardware with an skr1.3 mainboard)

https://youtu.be/Ja0v8Jaspes

Here are some of the quick notes and tips that should help:
1. Use the https://www.thingiverse.com/thing:3991855 from Beegeemon to print the base and duct. print both with 'Support only to build-plate' enabled. (NOT everywhere)
2. Follow the excellent build instructions from Roboscar here https://www.thingiverse.com/thing:2841443. Including the software changes. Note that some of the lines are different in Marlin 2.0. (but should be easy to figure out the equivalents)
3. I used the PIN29 mod on the stock Ender 3 board to get this working. Its easier to turn th