ROS Integration pages are missing references to ros_gz_image & ros_gz_pointcloud

[SteveMacenski]

ros2_integration.md is missing references to ros_gz_image and ros_gz_pointcloud packages and their uses.

The ros_gz_sim_demos also have irregular use of them (or have TODOs in the readme to include). This would be good to document the behavior of and then update the reference examples to use them fully for others to use as a template.

[azeey]

ros_gz_pointcloud has been broken for a while. I think instead of trying to fix it, we should use the new way of creating ros_gz bridges as ROS components alongside Gazebo (gazebosim/ros_gz#544) and then document how to use that.