Just dropping a brief note to comment on a couple things here:
First yes, that’s the newer docs plugin which we intend to replace the old. We are using it here on meta for our documentation where it’s serving our purposes much better. It allows you to define the sections and pages that show in the sidebar navigation in the order that you want.
It’s still experimental in the sense that it hasn’t quite reached the maturity we think it needs to be widely supported – the main thing that’s needed is a more user friendly way to manage the indices that back the navigation in the sidebar.
Supporting deeper nesting is something that I expect we’ll do at some point as well.
We aren’t investing too heavily in the plugin at this very moment, but it is the foundation on which I expect we’ll be building more of these features when they become a priority and/or in collaboration with partners.