docs/library/machine.SPI.rst: Add example SPI usage.
docs/library/machine.SPI.rst: Add example SPI usage.

Signed-off-by: Michael Buesch <6b0d31c0d563223024da45691584643ac78c96e8@bues.ch>

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython