You can create custom index pages that override the automatically generated ones.

Here's how. Create a page at the same level as the index page, and give it a name attribute whose value is index. Put whatever you like in the page body. Fargo will notice that it's there before creating an automatic index page and will skip it.

This site, the Fargo Docs site, uses a custom index page at the top level. You can find a link to the OPML source for this site on this page.