More control over next

Lots of our docs have a "flow" and once you get to the end of the flow you should "go back" to where you started. Look at [this](https://github.com/elastic/logstash/pull/11033#discussion_r330537177). But some combination of docbook and our nav structure locks us in to "next" being the next page in the book. But in these "flows" the "next" link should really be more of an "up" link or a "back to where I came from link".