Generate page index and provide method for relative access

[ethanblake4]

For docs we need to generate a tree structure of all the pages that is accessible by Jinja. The leaves should have the page metadata like title and URL. We also need to be able to define the order of the pages.

We also need to provide relative access; requirements currently are only the next and previous page relative to the current one, but if it's easy to provide generalized relative access then we can do that.

We need a way to define the 'root' of this relative access list so that we don't end up falling off the entire Docs page via a Previous link on the first docs page or a Next link on the last docs page.