Website should be versioned

[mattwthompson]

When the SMIRNOFF spec was housed in the toolkit docs, it was possible to get it as a snapshot in time via the version of the toolkit. (This was not a perfect solution, but it was good enough as an approximation.) It's not clear how to do that with the new site - i.e. https://openforcefield.github.io/standards/standards/smirnoff/ vs https://open-forcefield-toolkit.readthedocs.io/en/0.10.1/users/smirnoff.html or https://open-forcefield-toolkit.readthedocs.io/en/0.7.1/smirnoff.html or