update marlin 2.1, add Probe Z offset

[JPT77]

Hi
I am a newbie, so please check my contribution.
Setting up autolevelling was a pain in the a*** because this guide was totally outdated.
What's still missing is a description of how to set the borders right.
I do not fully understand this part, ie it does not work yet as expected.

this was done with Marlin 2.1.2.2 on two Creality configs that do not yet exist in the repo.
Should I already post them even if I am not finished yet?

[sjasonsmith]

Suggested change

You should be able to successfully print a small object at the center of the bed with bed leveling turned off. It's very important to verify that your `Configuration.h` settings make this possible before trying to bring up UBL. Most problems bringing up the UBL Bed Leveling system occur when this step has been ignored. Please pay particular attention to your `Z_PROBE_OFFSET_FROM_EXTRUDER` value (`NOZZLE_TO_PROBE_OFFSET` in Marilin 2.1). Usually it's best to home the Z-Axis in the center of the bed. But wherever you decide to home, the Z value reported on the LCD (or with [`M114`](/docs/gcode/M114.html)) should be _very_ close to 0.0 mm when the nozzle is just touching the bed. Failure to calibrate `Z_PROBE_OFFSET_FROM_EXTRUDER` properly will result in dimensional errors in your printed parts.

You should be able to successfully print a small object at the center of the bed with bed leveling turned off. It's very important to verify that your `Configuration.h` settings make this possible before trying to bring up UBL. Most problems bringing up the UBL Bed Leveling system occur when this step has been ignored. Please pay particular attention to your `Z_PROBE_OFFSET_FROM_EXTRUDER` value (`NOZZLE_TO_PROBE_OFFSET` in Marlin 2.1). Usually it's best to home the Z-Axis in the center of the bed. But wherever you decide to home, the Z value reported on the LCD (or with [`M114`](/docs/gcode/M114.html)) should be _very_ close to 0.0 mm when the nozzle is just touching the bed. Failure to calibrate `Z_PROBE_OFFSET_FROM_EXTRUDER` properly will result in dimensional errors in your printed parts.

[sjasonsmith]

Perhaps @thisiskeithb or @ellensp could look this over and suggest whether they think the changes are worthwhile.