add thebelab buttons to the tutorials

doc/source/tutorial/discretize.rst

doc/source/tutorial/faq.rst

doc/source/tutorial/first_steps.rst

doc/source/tutorial/graphene.rst

doc/source/tutorial/kpm.rst

doc/source/tutorial/magnetic_field.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/superconductors.rst