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_3.py

@@ -0,0 +1,11 @@
+from gpiozero import Button
+from signal import pause
+
+def say_hello():
+    print("Hello!")
+
+button = Button(2)
+
+button.when_pressed = say_hello
+
+pause()