Changes default ground color back to dark grey (#2164)

# Description

This MR fixes the ground color to the original dark grey instead of a
grey-ish blue shade. The change was introduced in #1791 and was
unintentional.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Screenshots

| Before | After |
| ------ | ----- |
|
![image](https://github.com/user-attachments/assets/655fd5b2-5b64-4e9f-adb1-c2b706b0aa13)
|
![image](https://github.com/user-attachments/assets/10ba218e-eff8-441b-9d5f-1705c90d7345)
|

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

---------

Signed-off-by: Mayank Mittal <12863862+Mayankm96@users.noreply.github.com>
Co-authored-by: Kelly Guo <kellyguo123@hotmail.com>

Author: Mayankm96

source/isaaclab/isaaclab/terrains/terrain_importer_cfg.py

@@ -68,9 +68,7 @@ class TerrainImporterCfg:
       This parameter is used only when the ``terrain_type`` is "plane" or "usd".
     """
 
-    visual_material: sim_utils.VisualMaterialCfg | None = sim_utils.PreviewSurfaceCfg(
-        diffuse_color=(0.065, 0.0725, 0.080)
-    )
+    visual_material: sim_utils.VisualMaterialCfg | None = sim_utils.PreviewSurfaceCfg(diffuse_color=(0.0, 0.0, 0.0))
     """The visual material of the terrain. Defaults to a dark gray color material.
 
     This parameter is used for both the "generator" and "plane" terrains.

source/isaaclab_rl/isaaclab_rl/__init__.py

@@ -17,5 +17,3 @@
 expect different input and output data structures, their wrapper classes are not compatible with each other.
 Thus, they should always be used in conjunction with the respective learning framework.
 """
-
-__all__ = ["sb3", "skrl", "rsl_rl", "rl_games"]