update .yml to add drude solutions

mkdocs.yml

@@ -34,6 +34,7 @@ nav:
   - Solutions:
     - Einstein model: '1_einstein_model_solutions.md'
     - Debye model: '2_debye_model_solutions.md'
+    - Drude model: '3_drude_model_solutions.md'
 
 theme:
   name: material