Add default introductory code example

Instead of having logic for a fallback in the backend, we're choosing a
default for the snippet file.

For tracks that have core exercises, we pick the first core exercise.
For tracks without these, we pick the first exercise listed in the config.

Note that we're aiming for 10 lines and a max-width of 40 columns.
This solution has 11 lines, so we may want to adjust it.

docs/SNIPPET.txt

@@ -0,0 +1,10 @@
+module HelloWorld exposing (..)
+
+helloWorld : Maybe String -> String
+helloWorld name =
+    case name of
+        Just name ->
+            "Hello, " ++ name ++ "!"
+
+        Nothing ->
+            "Hello, World!"