Following the steps on https://Fortress documentation on ros gazebo bridge I have noticed that the installed ros bridge is named
ros_gz_bridge and not ros_ign_bridge.
I think you need to update the documentation and change
ros2 run ros_ign_bridge parameter_bridge /TOPIC@ROS_MSG@IGN_MSG and its following example to
ros2 run ros_gz_bridge ... to avoid not found package error.