This plugin allows you to add RSS feeds to all your folders.
Also a generic RSS feed with all posts from all folders is available just add /rss to your base url. The title and description of this generic RSS feed can be set in the plugin settings.
Modified version of the original plugin, since my instance does not generate navigation.txt and therefore RSS is not generated at all or only links to the homepage.
My version takes the structure.txt file where the site structure is cached. But unfortunately it lacks, for example, a description (the page name is used).
Also RSS per folder is missing in this version.
The logic for generating RSS XML is in rssBuilder.php, which uses the PHP DOM.