ROS Integration pages are missing references to ros_gz_image & ros_gz_pointcloud

`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.