Docsy has its own user guide (using Docsy, of course!) with lots more information about using the theme, which you can find at https://docsy.dev/. Alternatively you can use Hugo to generate and serve a local copy of the guide (also useful for testing local theme changes), making sure you have installed all the prerequisites listed above:
git clone --recurse-submodules --depth 1 https://github.com/google/docsy.git
hugo server --themesDir ../..
Note that you need the themesDir flag when running Hugo because the site files are inside the theme repo.