Update Livebook setup instructions

[jonatanklosko]

We tell users to set XLA_TARGET which tells the Elixir's tensor library to use the GPU. We changed the default Docker image for the space to use CUDA 12, so this updates the docs accordingly.

[jonatanklosko]

Actually, I've just realised that it's probably best for us to set that env var in the Dockerfile directly. I will open a new PR later!