Summary

Add the ability to customize the TOC with links to pages that don't exist within the current content set. The goal is be able to interweave internal (elastic.co/docs/*) and external links to content into the navigation structure of a content set. This will allow us to, for example, link to API docs from within a narrative TOC.

If the content being linked to is within a different content set, the TOC should update to reflect the move to a new content set.