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_1.py

@@ -0,0 +1,8 @@
+from gpiozero import DistanceSensor
+from time import sleep
+
+sensor = DistanceSensor(23, 24)
+
+while True:
+    print('Distance to nearest object is', sensor.distance, 'm')
+    sleep(1)