docs: add labels to tutorial
update docstrings of poisson.py, poisson_short_syntax.py examples

- add mutual references, tutorial reference
- remove comments in examples/diffusion/poisson.py as their text is in the
  tutorial
update docstring of poisson_periodic_boundary_condition.py example
update docstring of poisson_field_dependent_material.py example
new examples/diffusion/poisson_neumann.py + test - closes #195