fix ray-tracing for conda site-package, find the right path

[KafuuChikai]

If you install genesis in a conda environment using:

pip install genesis-world

the package will be installed under ${your_path_to_anaconda3}/envs/${your_env}/lin/python3.x/site-packages directory.

In genesis/vis/raytracer.py, the following line (Line 14):

LRP_PATH = os.path.join(miscu.get_src_dir(), "ext/LuisaRender/build/bin")

will result in an incorrect path.

To address this issue, it is recommended to set the appropriate environment variable directly:

export LRP_PATH=your/path/to/LuisaRender/build/bin

And then import LuisaRenderPy from the path specified by the environment variable LRP_PATH

This will prioritize the use of the environment variable.

Please add this information to the documentation.