Write documentation for molecule generators

[joshkamm]

• [x] Numpydoc style docstrings
  • https://www.sphinx-doc.org/en/master/usage/extensions/example_numpy.html
  • https://numpydoc.readthedocs.io/en/latest/format.html#docstring-standard
  • Does PEP604 have a backport to python 3.7?
• [x] Type hints

[runxuanjiang]

Docs updated for lignin, xorgate, alkanes