Improve section-as-root navigation

[chalin]

Originally posted by @jaydeluca in https://github.com/open-telemetry/opentelemetry.io/pull/8344#discussion_r2499141109

I like that it declutters the navigation, especially when there are so many options now.

I don't like that it requires leaving the current page in order to get back to a state where you can view the entire navigation tree (when you click on the "Collector ^" link). I don't see that as a blocker, but it was unexpected behavior from my perspective.

@jaydeluca - I'm not sure I can do anything about showing the full sidenav content without leaving the page, but would this extra element (the parent page link at the top) help make the navigation more intuitive?

Oh, I'm thinking that maybe you want this to be a different kind of root. Let me try something and post back here.

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Upstreamed to https://github.com/google/docsy/issues/2418.