You're reading the documentation for an older, but still supported, version of ROS 2. For information on the latest version, please have a look at Iron.
Migrating from ROS 1 to ROS 2
These guides show how to convert existing ROS 1 packages to ROS 2. If you are new to porting between ROS 1 and ROS 2, it is recommended to read through the guides in order.
There are also some automatic conversion tools that exist, though they are not exhaustive:
Launch File migrator that converts a ROS 1 XML launch file to a ROS 2 Python launch file: https://github.com/aws-robotics/ros2-launch-file-migrator
Amazon has made their tools for porting from ROS 1 to ROS 2 available at: https://github.com/awslabs/ros2-migration-tools/tree/master/porting_tools
rospy2 Python project to automatically convert rospy calls to rclpy calls