Custom TOC/nav links

[bmorelli25]

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.

[theletterf]

Agree. I think the external links path is the simplest, also to avoid crosslink resolution in the nav:

• You define an external link like this:

toc:
  - link: https://elastic.co/docs/something

It would render like a nav item, but with an external link indicator, like we added here: #1096