dev env: deprioritize nix

[conorsch]

We currently document two workflows for getting set up with the repo:

• nix automagic for developers https://guide.penumbra.zone/dev/dev-env
• manual package installation for building from source https://guide.penumbra.zone/dev/build

During recent discussion we decided to prioritize Rust-centric tooling like cargo for the dev guide. We can preserve the nix docs for those in the know, but let's kick them down to the bottom, and link to them from the header. That way, the non-nix, manual steps are covered first, with an escape hatch for nix-pilled folks who want to fast-forward.