Document hierarchy of molecular representation classes

[t-kimber]

Have class-based documentation with hierarchy. In Sphinx docs. See example: https://open-forcefield-toolkit.readthedocs.io/en/0.10.0/topology.html Source: https://open-forcefield-toolkit.readthedocs.io/en/0.10.0/_sources/topology.md.txt

[corey-taylor]

KinoML object model:

https://docs.google.com/presentation/d/1jVBlZBlczQYDBcWt8Q266RISLf-aViIUrvHFZlRltek/edit#slide=id.gcedb739bd3_0_173