Handbook pages should have "next" and "previous" links at the bottom

[samuelsidler]

As the summary says. If you're going through the handbook(s) and going through the step-by-step process of learning how to write a theme/plugin, when you finish a page ("chapter"?), you'll want to go to the next page. For ease of use, we should add a "next" link to the bottom of each page and perhaps a "previous" link.