A mountainous environment and a foggy environment have been added.

[serkanMzlm]

🎉 New feature

Summary

• Fog has been added to the mountainous environment.
• This example may be useful for DEM(Digital Elevation Model) usage.

Test it

• The repository needs to include the 'examples' directory in GZ_SIM_RESOURCE_PATH. For example

  export GZ_SIM_RESOURCE_PATH=/home/${USER}/gz-sim/examples
  gz sim -r mountain_fog.sdf 

Note: The collision part of the world can be optionally enabled. It is closed by default.

[arjo129]

Can you sign-off your commits: https://github.com/gazebosim/gz-sim/pull/2436/checks?check_run_id=25897514741