Document `cuda-libraries`; use of cudatoolkit in CUDA 12+ world

I [answered](https://github.com/conda-forge/admin-requests/pull/1680#issuecomment-3435646664) a question today that made me dig through some issues about the CUDA 12 bring-up again, and noticed that there's a convenience package `cuda-libraries` that I had never seen or heard of. [Feedstock](https://github.com/conda-forge/cuda-libraries-feedstock), [staged-recipes PR](https://github.com/conda-forge/staged-recipes/pull/23073)

We should document this, plus how to set up the equivalent of the old cudatoolkit in a user environment (not recipe).

Xref: #1927

CC @conda-forge/cuda