finish formatting exercises

docs/10_xray.md

docs/10_xray_solutions.md

docs/11_nearly_free_electron_model.md

docs/11_nearly_free_electron_model_solutions.md

docs/12_band_structures_in_higher_dimensions.md

docs/12_band_structures_in_higher_dimensions_solutions.md

docs/13_semiconductors.md

docs/13_semiconductors_solutions.md

docs/14_doping_and_devices.md

docs/14_doping_and_devices_solutions.md

docs/4_sommerfeld_model.md

docs/4_sommerfeld_model_solutions.md

docs/5_atoms_and_lcao.md

docs/5_atoms_and_lcao_solutions.md

docs/6_bonds_and_spectra.md

docs/6_bonds_and_spectra_solutions.md

docs/7_tight_binding.md

docs/7_tight_binding_model_solutions.md

docs/8_many_atoms_solutions.md

docs/9_crystal_structure.md