rename tutorials to give more descriptive URLs

doc/source/tutorial/discretizer.rst

doc/source/tutorial/first_steps.rst

doc/source/tutorial/graphene.rst

doc/source/tutorial/index.rst

doc/source/tutorial/kpm.rst

doc/source/tutorial/operators.rst

doc/source/tutorial/plotting.rst

doc/source/tutorial/spectrum.rst

doc/source/tutorial/spin_potential_shape.rst

doc/source/tutorial/superconductor.rst