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/led_variable_brightness.py

@@ -0,0 +1,12 @@
+from gpiozero import PWMLED
+from time import sleep
+
+led = PWMLED(17)
+
+while True:
+    led.value = 0  # off
+    sleep(1)
+    led.value = 0.5  # half brightness
+    sleep(1)
+    led.value = 1  # full brightness
+    sleep(1)