Update Documentation for Spawn a Gazebo model from ROS 2

[Amronos]

🦟 Bug fix

Updates the documentation for Spawn a Gazebo model from ROS 2 as described here, alongside another change.

Summary

Updates the documentation for Spawn a Gazebo model from ROS 2 as described here, alongside changing gz_spawn_model.launch.py to ros_gz_spawn_model.launch.py in the command example.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.