Add default introductory code example (#516)

Instead of having logic for a fallback in the backend, we're going to
actually add a snippet.

docs/SNIPPET.txt

@@ -0,0 +1,10 @@
+prices = {'apple': 0.75, 'egg': 0.50}
+cart = {
+  'apple': 1,
+  'egg': 6
+}
+
+bill = sum(prices[item] * cart[item]
+           for item in cart)
+
+print(f'I have to pay {bill:.2f}')