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 Setting up autolevelling was a pain in the a*** because this guide was totally outdated.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]

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