Enable documentation testing

@embray @eteq @cdeil - how do we enable the testing of examples in documentation? Should it just work? (If so, it doesn't, so this may indicate a bug in the testing framework)
I don't understand the phrasing before this example: " We consider a large array of zeros with the shape 11x10 and a small array of ones with a shape of 3x4" -> this doesn't seem right?