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/button_camera_2.py

@@ -0,0 +1,18 @@
+from gpiozero import Button
+from picamera import PiCamera
+from datetime import datetime
+from signal import pause
+
+left_button = Button(2)
+right_button = Button(3)
+camera = PiCamera()
+
+def capture():
+    datetime = datetime.now().isoformat()
+    camera.capture('/home/pi/%s.jpg' % datetime)
+
+left_button.when_pressed = camera.start_preview
+left_button.when_released = camera.stop_preview
+right_button.when_pressed = capture
+
+pause()