Add Z-offset information for Manual Bed Levelling section

[drf5n]

teachingtechYT.github.io/calibration.html

Lines 267 to 270 in 0186e1c

	<h2>Manual Levelling Procedure</h2>
	<p>There are many techniques available, but a common one is to move the nozzle to the various corners of the bed, turning the levelling knobs until a standard piece of office paper can just fit between the bed and nozzle. A 0.1mm feeler gauge can be used, but make sure it doesn't have any oil on it that will contaminate the bed surface. Typically, this procedure is done with the bed at printing temperature (essential), and the nozzle close to printing temperature - just cool enough to prevent filament oozing out (optional).</p>
	<p>It is common to follow up with a first layer calibration print, and 'live level' the bed by continuing to adjust the knobs when the print is under way.</p>
	<p>This process is depicted in detail in the video above, and a gcode generator is provided at the bottom of the page to generate a suitable test print.</p>

The current doc leaves the Z-offset setting to the autoprobe section, and does not cover adjustment in of the ZOffset for the manual case.

If you do the homing, and then level the bed to a paper thickness/0.1mm, then the bed is level, but 0.1mm below the nozzle and after homing a M114 would report something like:

X:0.00 Y:0.00 Z:0.00 E:0.00 Count X:0 Y:0 Z:0

and the first layer calibration will print the 0.2mm layer with the nozzle at 0.3mm above the bed and potentially be a bit porous and in need of adjustment on all corners to bring the bed the 0.1mm up to touch the nozzle at Z=0.0

If after you do the Z=0.1mm paper leveling you do a M206 Z0.1 then with a M114 printer will report X:0.00 Y:0.00 Z:0.10 E:0.00 Count X:0 Y:0 Z:0 post homing, and the 0.20 first layer calibration will print 0.1mm higher than the paper at 0.20mm above the bed, as expected. It still might take some adjustment, but at least you are centered on the expected layer height of 0.2mm above the bed.

Maybe one should do:

M206 Z0 ; reset homing vertical offset
G92 ; home to 0,0,0
M18 ; disable steppers
; manually level the bed with a 0.1mm paper
M206 Z0.1 ; set the offset for the 0.1mm paper
G92 ; re-home to 0,0,0.1  

Run 1st layer 0.20mm calibration print and tune to layer height & quality.