Convert recipe examples to includes

Makes it much easier to test things - no copying'n'pasting just run the
examples straight from the dir (after wiring stuff up)

docs/examples/robot_2.py

@@ -0,0 +1,9 @@
+from gpiozero import Robot, DistanceSensor
+from signal import pause
+
+sensor = DistanceSensor(23, 24, max_distance=1, threshold_distance=0.2)
+robot = Robot(left=(4, 14), right=(17, 18))
+
+sensor.when_in_range = robot.backward
+sensor.when_out_of_range = robot.stop
+pause()