Do not constantly reconstitute document.

I was reconstituting the document on each click of forward and back causing the
instance state to become out of sync with the DOM.
Add a module to increment example.

I added a module that will implement the logic of the increment example.
It encapsulates the extraction of the number from the URL path and the
generation of the previous and next link URLs. This illustrates how to
simplify a stencil by pushing the complexity into a module.