Fill out developer docs for ParameterType, Molecule.to/from_X, and forcefield directories

**Is your feature request related to a problem? Please describe.**
We're missing important components of our developer docs, namely around how to handle modular design features that might confuse contributors. See the TODOs at this link: https://github.com/openforcefield/openff-toolkit/blame/efb6985596605f986319a70bfd64037723b0d8f6/docs/developing.rst#L152