Update with display.root_group for CircuitPython 9 compatibility

[prcutler]

This PR replaces display.show with display.root_group in the 4 example files and the README

[dhalbert]

I too am rather baffled by

Warning, treated as error:
/home/runner/work/Adafruit_CircuitPython_SPD1656/Adafruit_CircuitPython_SPD1656/adafruit_spd1656.py:docstring of adafruit_spd1656.SPD1656:15:'any' reference target not found: displayio.FourWire
Error: Process completed with exit code 2.

[FoamyGuy]

I think this error is somehow due to the superclass that is being extended by this

Adafruit_CircuitPython_SPD1656/adafruit_spd1656.py

Line 51 in 1f9014e

class SPD1656(displayio.EPaperDisplay):

I'm not entirely sure why that error comes up. I think sphinx is using Blinka_DisplayIO for it's knowledge of displayio which doesn't have the latest chagnes / split up from the core I think. So perhaps that ties into this somehow.

In any case I think EPaperDisplay was moved with that recent change as well so this line should probably be updated to:

class SPD1656(epaperdisplay.EPaperDisplay): 

I tested out this doc build locally and found that if I add epaperdisplay (or displayio if that isn't changed) to the list of mock modules here in the docs conf.py file: https://github.com/adafruit/Adafruit_CircuitPython_SPD1656/blob/main/docs/conf.py#L30 it will allow the docs to build successfully.

[FoamyGuy]

I added one commit to change this to use the new epaperdisplay module.

This looks good to me.