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` :)

doc/languages-frameworks/rust.section.md

···
 
 For other versions such as daily builds (beta and nightly),
 use either `rustup` from nixpkgs (which will manage the rust installation in your home directory),
-or use Mozilla's [Rust nightlies overlay](#using-the-rust-nightlies-overlay).
+or use a community maintained [Rust overlay](#using-community-rust-overlays).
 
 ## Compiling Rust applications with Cargo {#compiling-rust-applications-with-cargo}
 
···
 
 To see that you are using nightly.
 
-## Using the Rust nightlies overlay {#using-the-rust-nightlies-overlay}
+## Using community Rust overlays {#using-community-rust-overlays}
 
-Mozilla provides an overlay for nixpkgs to bring a nightly version of Rust into scope.
-This overlay can _also_ be used to install recent unstable or stable versions
-of Rust, if desired.
+There are two community maintained approaches to Rust toolchain management:
+- [oxalica's Rust overlay](https://github.com/oxalica/rust-overlay)
+- [fenix](https://github.com/nix-community/fenix)
 
-### Rust overlay installation {#rust-overlay-installation}
+Oxalica's overlay allows you to select a particular Rust version and components.
+See [their documentation](https://github.com/oxalica/rust-overlay#rust-overlay) for more
+detailed usage.
 
-You can use this overlay by either changing your local nixpkgs configuration,
-or by adding the overlay declaratively in a nix expression,  e.g. in `configuration.nix`.
-For more information see [the manual on installing overlays](#sec-overlays-install).
+Fenix is an alternative to `rustup` and can also be used as an overlay.
 
-#### Imperative rust overlay installation {#imperative-rust-overlay-installation}
+Both Oxalica's overlay and fenix better integrate with nix and cache optimizations.
+Because of this and ergonomics, either of those community projects
+should be preferred to the Mozilla's Rust overlay (nixpkgs-mozilla).
 
-Clone [nixpkgs-mozilla](https://github.com/mozilla/nixpkgs-mozilla),
-and create a symbolic link to the file
-[rust-overlay.nix](https://github.com/mozilla/nixpkgs-mozilla/blob/master/rust-overlay.nix)
-in the `~/.config/nixpkgs/overlays` directory.
+### How to select a specific rustc and toolchain version {#how-to-select-a-specific-rustc-and-toolchain-version}
 
-```ShellSession
-$ git clone https://github.com/mozilla/nixpkgs-mozilla.git
-$ mkdir -p ~/.config/nixpkgs/overlays
-$ ln -s $(pwd)/nixpkgs-mozilla/rust-overlay.nix ~/.config/nixpkgs/overlays/rust-overlay.nix
+You can consume the oxalica overlay and use it to grab a specific Rust toolchain version.
+Here is an example `shell.nix` showing how to grab the current stable toolchain:
+```nix
+{ pkgs ? import <nixpkgs> {
+    overlays = [
+      (import (fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz"))
+    ];
+  }
+}:
+pkgs.mkShell {
+  nativeBuildInputs = with pkgs; [
+    pkg-config
+    rust-bin.stable.latest.minimal
+  ];
+}
 ```
 
-### Declarative rust overlay installation {#declarative-rust-overlay-installation}
+You can try this out by:
+1. Saving that to `shell.nix`
+2. Executing `nix-shell --pure --command 'rustc --version'`
 
+As of writing, this prints out `rustc 1.56.0 (09c42c458 2021-10-18)`.
+
+### Rust overlay installation {#rust-overlay-installation}
+
+You can use this overlay by either changing your local nixpkgs configuration,
+or by adding the overlay declaratively in a nix expression,  e.g. in `configuration.nix`.
+For more information see [the manual on installing overlays](#sec-overlays-install).
+
+### Declarative Rust overlay installation {#declarative-rust-overlay-installation}
+
+This snippet shows how to use oxalica's Rust overlay.
 Add the following to your `configuration.nix`, `home-configuration.nix`, `shell.nix`, or similar:
 
 ```nix
 { pkgs ? import <nixpkgs> {
     overlays = [
-      (import (builtins.fetchTarball https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz))
+      (import (builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz"))
       # Further overlays go here
     ];
   };
···
 ```
 
 Note that this will fetch the latest overlay version when rebuilding your system.
-
-### Rust overlay usage {#rust-overlay-usage}
-
-The overlay contains attribute sets corresponding to different versions of the rust toolchain, such as:
-
-* `latest.rustChannels.stable`
-* `latest.rustChannels.nightly`
-* a function `rustChannelOf`, called as `(rustChannelOf { date = "2018-04-11"; channel = "nightly"; })`, or...
-* `(nixpkgs.rustChannelOf { rustToolchain = ./rust-toolchain; })` if you have a local `rust-toolchain` file (see https://github.com/mozilla/nixpkgs-mozilla#using-in-nix-expressions for an example)
-
-Each of these contain packages such as `rust`, which contains your usual rust development tools with the respective toolchain chosen.
-For example, you might want to add `latest.rustChannels.stable.rust` to the list of packages in your configuration.
-
-Imperatively, the latest stable version can be installed with the following command:
-
-```ShellSession
-$ nix-env -Ai nixpkgs.latest.rustChannels.stable.rust
-```
-
-Or using the attribute with nix-shell:
-
-```ShellSession
-$ nix-shell -p nixpkgs.latest.rustChannels.stable.rust
-```
-
-Substitute the `nixpkgs` prefix with `nixos` on NixOS.
-To install the beta or nightly channel, "stable" should be substituted by
-"nightly" or "beta", or
-use the function provided by this overlay to pull a version based on a
-build date.
-
-The overlay automatically updates itself as it uses the same source as
-[rustup](https://www.rustup.rs/).