create bed leveling documentation. fixes #120

[crazybilly]

Grabbed the documentation that @SnowByteK wrote on #120 and pushed it into a doc page, linked to it form the home page.

[mavanmanen]

Testing this results in the following error for me:
!! SAVE_CONFIG section 'stepper_z' option 'position_endstop' conflicts with included value

[clutchkick512]

Testing this results in the following error for me: !! SAVE_CONFIG section 'stepper_z' option 'position_endstop' conflicts with included value

Go into printer.base.cfg- under stepper z comment out the old position_endstop then paste the new one under it on the next line. save and restart, worked for me.

[mavanmanen]

Testing this results in the following error for me: !! SAVE_CONFIG section 'stepper_z' option 'position_endstop' conflicts with included value

Go into printer.base.cfg- under stepper z comment out the old position_endstop then paste the new one under it on the next line. save and restart, worked for me.

Can confirm that works yes, except I put it in the printer.cfg instead.

[konradzawadka]

What about Step 2 and 3, are you doing it also with room temperature? I always go with profile temperature like 255/98 or 240/60.