WIP: add flake devShell

Adds a flake so you can use `nix develop` to enter a devShell where
you can use `hugo` to regenerate the site.

Unfortunately this does not quite work yet: it makes various incorrect
changes to the site, such as replacing links in the menu with `/`.

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1726937504,
+        "narHash": "sha256-bvGoiQBvponpZh8ClUcmJ6QnsNKw0EMrCQJARK3bI1c=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9357f4f23713673f310988025d9dc261c20e70c6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,18 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+  outputs = { self, nixpkgs }: {
+    devShells."x86_64-linux".default =
+      let 
+        pkgs = nixpkgs.legacyPackages."x86_64-linux";
+      in
+        pkgs.mkShell {
+          packages = [
+            pkgs.hugo
+            pkgs.go
+            pkgs.nodejs
+          ];
+        };
+  };
+}