update .xml file

mkdocs.yml

@@ -35,6 +35,7 @@ nav:
     - Einstein model: '1_einstein_model_solutions.md'
     - Debye model: '2_debye_model_solutions.md'
     - Drude model: '3_drude_model_solutions.md'
+    - Sommerfeld model: '4_sommerfeld_model_solutions.md'
     # - Sommerfeld model: '4_sommerfeld_model_solutions.md'
     # - LCAO model: '5_atoms_and_lcao_solutions.md'
     # - Bonds and spectra: '6_bonds_and_spectra_solutions.md'