Add more documentation to environmental_sensor.sdf example world (backport #2836)

[mergify]

🎉 New feature

Closes #

Summary

Not sure if we have tutorial for environment sensors but it took me some time to understand how to use the GUI plugins associated with the environment gz-sim systems. We should probably add a tutorial for this. In the mean time, I added some doc to the example world file.

I also moved the EnvironmentalSystem system below the EnvironmentPreload otherwise it prints out a confusing error message:

error] [EnvironmentalSensorSystem.cc:460] No sensor data loaded

This message is actually harmless because it will retry on PostUpdate and succeeds once EnvironmentPreload system is loaded. I rearranged the order of the systems so that the data is ready before loading the EnvironmentalSystem system

Test it

Follow the instructions in the updated environmental_sensor.sdf world and you should point cloud visualization in the 3D scene:

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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.

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This is an automatic backport of pull request #2836 done by [Mergify](https://mergify.com).

[ahcorde]

@iche033 can you merge this one ?