Improve documentation on how to use the ros_gz_sim/create node.

[azeey]

Desired behavior

The use of the ros_gz_sim/create node is not well documented. The README shows how to run the node from the command line, but doesn't have examples of its use in a launch file. We should also add an example of it in the ros_gz_project_template repo.

[Amronos]

I think this can be closed as the gz_spawn_model.launch.py and gz_spawn_model.launch launch files act as an example of how to use the node.