docs: improve, clean up Rust language advice

This stems from a discussion [here](https://discourse.nixos.org/t/what-rust-overlay-do-you-use-and-why-advice-appreciated/15412)

I removed an entire section because I feel like that duplicated
Mozilla's original instructions on how to consume the overlay.

The goal here is to simply the "getting started with Rust" in a nix or
NixOS environment.

I will try to do some follow up work to update the code snippets and
output. nightly is on `1.57.0-nightly` :)

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