Updated the orbitals example and notebook docs

Author: avmarchenko

docs/source/index.rst

@@ -35,6 +35,7 @@ method to get started with visualization is through the `Jupyter notebook`_.
     notebooks/01_basics.ipynb
     notebooks/02_xyz.ipynb
     notebooks/03_orbitals.ipynb
+    notebooks/04_cluster_extraction.ipynb
     notebooks/nwchem.ipynb
     notebooks/qe.ipynb