chapter hierarchy

I started to create my first book, but after reading the documentation and playing around with it, I still can't figure out how to create a chapter hierarchy.
The documentation page has main chapters (1, 2, 3 ...) and sub chapters (1.1, 1.2, 1.3 ...). How do I do that? From the article page I just seem to have the option to add the page as chapter to the book itself, making it a new top level chapter.

To do this, the chapters must also be created as books. You can move them to your Book forum if you need to. You then add the (chapter) books into your main book.