docs(mkdocs): add shell.nix for python doc deps

shell.nix

@@ -0,0 +1,11 @@
+{pkgs ? import <nixpkgs> {}}:
+with pkgs;
+  mkShell {
+    name = "komorebi";
+
+    buildInputs = [
+      python311Packages.mkdocs-material
+      python311Packages.mkdocs-macros
+      python311Packages.setuptools
+    ];
+  }