Update mkdocs.yml

mkdocs.yml

@@ -35,6 +35,7 @@ nav:
     - Einstein model: '1_einstein_model_solutions.md'
     - Debye model: '2_debye_model_solutions.md'
     - Tight-binding model: '7_tight_binding_model_sol.md'
+    - Many atoms per unit cell: '8_many_atoms_sol.md'
 
 theme:
   name: material