add the solutions to the TOC

mkdocs.yml

@@ -37,7 +37,7 @@ nav:
     - 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'
+    - 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'