docs: switch to book-theme

shell.nix

@@ -1,18 +1,21 @@
-{ pkgs ? import <nixpkgs> {} }:
+{
+  pkgs ? import <nixpkgs> { },
+}:
 
 let
-  unstable = import (fetchTarball https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz) { };
+  unstable = import (fetchTarball "https://channels.nixos.org/nixos-unstable/nixexprs.tar.xz") { };
 in
 pkgs.mkShell {
   packages = with pkgs; [
     python312
     unstable.uv
     gh
+    graphviz
   ];
 
   shellHook = ''
-  if [[ -d .venv/ ]]; then
-    source .venv/bin/activate
-  fi
+    if [[ -d .venv/ ]]; then
+      source .venv/bin/activate
+    fi
   '';
-}
+}