add flake output for the website root

nix/pkgs/website-root/default.nix

@@ -0,0 +1,28 @@
+# Keep sorted
+{ inputs
+, lib
+, mdbook
+, stdenv
+}:
+
+stdenv.mkDerivation {
+  name = "website-root";
+
+  src = let filter = inputs.nix-filter.lib; in filter {
+    root = inputs.self;
+
+    # Keep sorted
+    include = [
+      "book"
+      "book.toml"
+    ];
+  };
+
+  buildPhase = ''
+    ${lib.getExe mdbook} build
+  '';
+
+  installPhase = ''
+    mv target/book $out
+  '';
+}