Add example to color octree by height or RGB, fixes #5966

[Mereshiya]

This PR addresses Issue #5966 by adding an example script to color octree leaf nodes based on the height (z-coordinate) or RGB colors of a point cloud. The script:

• Creates a point cloud from synthetic data or a .ply file.
• Supports height-based coloring (jet, hot, viridis, cool) with '4' key toggling.
• Supports RGB colors (or default gray) with '5' key to toggle modes.
• Colors octree leaf nodes by averaging point colors.
  Tested with Open3D 0.16.0.

Fixes #5966

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[Mereshiya]

@syncle Can you please help in reviewing it