Learn website doesn't tell you how to get the notebooks

[keflavich]

The Learn notebooks, when rendered, don't point back to the notebooks.

In order for students to do the exercises, they therefore have to track down the associated notebook on github. While this is a fine exercise to require of students, it would be nice if the index also linked to the parent notebooks.

This issue is particularly relevant to #405 (and other exercises-related topics).