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/music_box.py

@@ -0,0 +1,18 @@
+from gpiozero import Button
+import pygame.mixer
+from pygame.mixer import Sound
+from signal import pause
+
+pygame.mixer.init()
+
+sound_pins = {
+    2: Sound("samples/drum_tom_mid_hard.wav"),
+    3: Sound("samples/drum_cymbal_open.wav"),
+}
+
+buttons = [Button(pin) for pin in sound_pins]
+for button in buttons:
+    sound = sound_pins[button.pin.number]
+    button.when_pressed = sound.play
+
+pause()