Add docs about custom toolchains

Author: surma

docs/src/content/docs/00_guides/42_custom_toolchains.md

@@ -0,0 +1,64 @@
+---
+title: Custom Toolchains
+---
+
+One way to use a custom rust toolchain with `crate2nix` is to
+import nixpkgs with an overlay for `rustc`.
+
+Here is an example flake using [fenix]:
+
+```nix
+
+{
+  description = "containix";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/24.05";
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    flake-utils.url = "github:numtide/flake-utils";
+    crate2nix.url = "github:nix-community/crate2nix";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      fenix,
+      crate2nix,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        toolchain = fenix.packages.${system}.stable.defaultToolchain;
+
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [
+            (final: prev: {
+              rustc = toolchain;
+              cargo = toolchain;
+            })
+          ];
+        };
+
+        crate2nix' = pkgs.callPackage (import "${crate2nix}/tools.nix") {};
+        cargoNix = crate2nix.appliedCargoNix {
+          name = "my-crate";
+          src = ./.;
+        };
+      in
+      {
+        packages = {
+          default = cargoNix.rootCrate.build;
+        };
+      }
+    );
+}
+
+```
+
+[fenix]: https://github.com/nix-community/fenix