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/distance_sensor_2.py

@@ -0,0 +1,10 @@
+from gpiozero import DistanceSensor, LED
+from signal import pause
+
+sensor = DistanceSensor(23, 24, max_distance=1, threshold_distance=0.2)
+led = LED(16)
+
+sensor.when_in_range = led.on
+sensor.when_out_of_range = led.off
+
+pause()