add documentation template

[ltalirz]

@danieleongari mentioned he will read through the code over the weekend and would be happy to work on a documentation during this time

This adds

• a documentation template for use with readthedocs.org (I will add it there once the PR is merged)
• an automatic API documentation of the code based on current docstrings
• a brief sphinx cheatsheet showing how to refer to classes, write math etc.

[ltalirz]

Thanks @peteboyd - after merging another PR with fixes, the documentation (still rather empty) is now available at https://lammps-interface.readthedocs.io

It would make sense to add you as a maintainer - for this simply register at readthedocs.org and let me know your username.

Any changes to the docs in the docs/ folder should be available on readthedocs.org within a few minutes delay.

Once @danieleongari has added more content, we can also add a link + badge to the README.