update .yml to add drude solutions

mkdocs.yml

@@ -34,6 +34,18 @@ nav:
   - Solutions:
     - 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'
+    # - LCAO model: '5_atoms_and_lcao_solutions.md'
+    # - Bonds and spectra: '6_bonds_and_spectra_solutions.md'
+    # - Tight-binding model: '7_tight_binding_solutions.md'
+    # - Many atoms per unit cell: '8_many_atoms_solutions.md'
+    # - Crystal structure: '9_crystal_structure_solutions.md'
+    # - X-ray diffraction: '10_xray_solutions.md'
+    # - Nearly free electron model: '11_nearly_free_electron_model_solutions.md'
+    # - Band structures in 2D: '12_band_structures_in_higher_dimensions_solutions.md'
+    # - Basic principles: '13_semiconductors_solutions.md'
+    # - Doping and devices: '14_doping_and_devices_solutions.md'
 
 theme:
   name: material