You're reading the documentation for a version of ROS 2 that has reached its EOL (end-of-life), and is no longer officially supported. If you want up-to-date information, please have a look at Iron.
Latest development (source)
If you plan to contribute directly to the latest ROS 2 development, you can install ROS 2 by building it from source or installing testing binaries. This will give you the latest bug fixes and features.
Building from source
The latest development does not go through the same rigorous testing as releases and is not recommended if you are looking for a stable version of ROS 2. Instead, choose a ROS distribution that has already been released.
Follow the links below for the latest setup instructions for your platform:
For keeping your source code up-to-date, see Maintain a source checkout.