build: add flake with shell

flake.nix

@@ -0,0 +1,25 @@
+
+{
+  description = "Shell for building rust-for-linux.com";
+  inputs.nixpkgs.url = "nixpkgs/nixos-24.05";
+  inputs.systems.url = "github:nix-systems/default";
+  inputs.flake-utils = {
+    url = "github:numtide/flake-utils";
+    inputs.systems.follows = "systems";
+  };
+
+  outputs =
+    { nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell { packages = with pkgs; [
+          python3
+          mdbook
+        ]; };
+      }
+    );
+}