This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
The usage of the two-factor authentication is very easy. You can enable each individual backend and frontend user to use the two-factor authentication process.
After the extension is installed, there is a new tab in the user settings where you can enable the YubiKey two-factor authentication.