switch questions from lists to h4

docs/12_band_structures_in_higher_dimensions.md

docs/1_einstein_model.md

docs/1_einstein_model_solutions.md

docs/2_debye_model.md

docs/2_debye_model_solutions.md

docs/3_drude_model.md

docs/3_drude_model_solutions.md

docs/4_sommerfeld_model.md

docs/4_sommerfeld_model_solutions.md

docs/8_many_atoms.md