Add tutorial for using DEM's for gazebo harmonic

> Unfortunately, the only tutorial available is from gazebo-classic found here: https://classic.gazebosim.org/tutorials?tut=dem&cat=build_world

> It is outdated but will hopefully get you in the right direction. When following the tutorial, you should be able to copy this section into a Gazebo Garden compatible `SDF` world: https://github.com/gazebosim/gazebo-classic/blob/1bef2dc573aa15ca8a56b4a7dadb9905cf978582/worlds/heightmap_dem.world#L8-L54

> If you'd like to contribute, feel free to open a PR for a newer tutorial on DEMs here: https://github.com/gazebosim/docs :smile:

_Originally posted by @jennuine in https://github.com/gazebosim/gz-sim/issues/235#issuecomment-1412848224_

Creating an issue to keep track of this request for a tutorial on how to use the new DEM feature in gazebo sim (Garden). Once I free some time up, I will try the suggestions above and if it works, I'll be happy to submit a PR.