Add a note about background textures

[MRiabov]

Hello. This is in relation to https://genesis-world.readthedocs.io/en/latest/user_guide/getting_started/visualization.html

While demo.py shows how to add a background texture, via:

 env_surface=gs.surfaces.Emission(
                emissive_texture=gs.textures.ImageTexture(
                    image_path="textures/indoor_bright.png",
                ),
            ),

It probably is worth noting this in the docs.

Cheers.

Edit: is there no way to add background texture (only color) to rasterizer renderer? Why? What if I want to use rasterizer and not raytracer (performance?)