#2198 Add Nix/OS install story

Description

## Title

Adds support for installing on Nix/NixOS.

## Summary

I run NixOS, so the standard setup script doesn't work for me because the system-level dependencies are not at the expected paths. This adds a reproducible Nix environment in the form of a flake that declares CUDA support and all system dependencies, including Python itself. I also added a quick note to the install docs.

## What changed

See above.

## Extra Notes

Not sure if we should assume CUDA as a dependency. We might want to define different common system/hardware configurations as their own outputs in the flake. But this works for me.