unlisted option to generate doc pages that don't get put on index

[machow]

From pairing with @schloerke, an unlisted option would benefit sites that want to document a lot of type annotations. For example, py-shiny wants users to be able to click through type annotations, but doesn't want to list them on the reference page. This is because there are a lot of them, and they live in various places. In a sense it's fairly low priority in the reference API, but useful supplementary info.

We could add a unlisted field to the quartodoc config, that renders the Auto pages, but does not create entries on the index page.