Feature request: Modular navigation indexes (Generate Diataxis pages from Home)

With the Diataxis scheme, the Home page should include sections for each of the four categories (see [template](https://discourse.ubuntu.com/t/documentation-home-page-template/25279)). IIUC, each of those category headings should link to a page that contains the summary line, along with a tree of links. That tree of links should mirror the corresponding branch in the navigation page (e.g. https://multipass.run/docs/how-to-guides).

These pages currently need to be created from discourse posts that repeat the information in the navigation pane. They need to be kept in sync manually, which is error prone. Since all the information needed to produce these pages is present in the post for the home page, it would be nice to have them generated automatically.