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/thermometer.py

@@ -0,0 +1,12 @@
+from gpiozero import MCP3008
+from time import sleep
+
+def convert_temp(gen):
+    for value in gen:
+        yield (value * 3.3 - 0.5) * 100
+
+adc = MCP3008(channel=0)
+
+for temp in convert_temp(adc.values):
+    print('The temperature is', temp, 'C')
+    sleep(1)