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/robot_buttons.py

@@ -0,0 +1,23 @@
+from gpiozero import Robot, Button
+from signal import pause
+
+robot = Robot(left=(4, 14), right=(17, 18))
+
+left = Button(26)
+right = Button(16)
+fw = Button(21)
+bw = Button(20)
+
+fw.when_pressed = robot.forward
+fw.when_released = robot.stop
+
+left.when_pressed = robot.left
+left.when_released = robot.stop
+
+right.when_pressed = robot.right
+right.when_released = robot.stop
+
+bw.when_pressed = robot.backward
+bw.when_released = robot.stop
+
+pause()