Strategy for nav headings that make sense in one context but not in another

## Summary

Slack thread: https://elastic.slack.com/archives/C05PP2LEC1X/p1740602224538949?thread_ts=1740532178.416139&cid=C05PP2LEC1X

Doc previews currently take all of the content and build it together. This means if you have reference content, release notes, and deprecations, it all gets built to one preview in the preview. For example:


In this screenshot, you'll notice that the release notes and deprecations nav headings are not helpful—hey're just the product name. That's because in the site-wide IA, these nav headings are nested under headings like "Release notes" and "Deprecations". Something like this:


## Question

What can we do to improve this experience where a navigation_heading makes sense in the context of the assembler build, but not in the context of the preview build?