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_3.py

@@ -0,0 +1,9 @@
+from gpiozero import LightSensor, PWMLED
+from signal import pause
+
+sensor = LightSensor(18)
+led = PWMLED(16)
+
+led.source = sensor.values
+
+pause()