Add example using `pixi add --no-install`

[matthewfeickert]

I (re)learned today that the --no-install option exists for pixi add. So you can do things like (on linux-64 with an NVIDIA GPU)

$ pixi init example && cd example
$ pixi workspace system-requirements add cuda 12
$ pixi add --no-install pytorch-gpu

and have a fully locked CUDA-enabled environment ready to go for distribution without having to have anything get installed on your machine.