Document how to set the rendering backend

Graphics backend compatibility issues are one of our most common sources of configuration pain still, and trying out different backends is one of the common recommended troubleshooting steps.

I believe you can set the WGPU_BACKEND environment variable to do so, but I'm not familiar enough with rendering to be entirely sure if that's true, or how to verify that it worked.

We should a) verify that this works b) decide where to put this sort of troubleshooting information c) document this approach.