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_shutdown.py

@@ -0,0 +1,11 @@
+from gpiozero import Button
+from subprocess import check_call
+from signal import pause
+
+def shutdown():
+    check_call(['sudo', 'poweroff'])
+
+shutdown_btn = Button(17, hold_time=2)
+shutdown_btn.when_held = shutdown
+
+pause()