There are three parts to the Frontity documentation.
docs.frontity.org is the generic documentation and where you should go if you want to deepen your conceptual understanding of how Frontity works and how a Frontity project should be implemented. As well as theoretical information, such as Frontity Architecture and Core Concepts, you can find many practical guides here.
api.frontity.org is the API reference. Once you've got the basics and the core concepts of working with Frontity nailed this is where you'll find detailed technical descriptions for the CLI and plugins available in Frontity. This is also the place where you should go if you want to learn more about the different Frontity packages, including themes.