Add docs about force field choices and enumerate all the possible small molecule force fields.

[IAlibay]

Users don't know what versions of different force fields they can use, there should be docs on where they can find out and what is allowed.

[mikemhenry]

I am not sure if we should point them to the openmmff docs, or we update the docs on settings class to list all of them.

We can then have a separate FAQ or other doc section that explains what force fields and what sort of systems users can expect to work.

[IAlibay]

If we can create the auto-generate list in docs that might be the best option, I don't think even openmmforcefields does that.