More better docs
Created by: PhilipVinc
This PR improves a few docstrings by adding links to openfermion, qutip and jax.
Mainly, this PR fixes the rest of our remaining documentation build errors (a few remain, but they are caused by flax so we can't really fix them directly). This is to get us to a day where we turn on strict building of docs, so that if people don't write proper docstrings in PR we notice.