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/light_sensor_2.py

@@ -0,0 +1,10 @@
+from gpiozero import LightSensor, LED
+from signal import pause
+
+sensor = LightSensor(18)
+led = LED(16)
+
+sensor.when_dark = led.on
+sensor.when_light = led.off
+
+pause()