Fix documentation build errors

[rkingsbury]

The documentation currently builds successfully but generates a number of warnings, such as

src/pyEQL/solution.py:docstring of pyEQL.solution.Solution:1: WARNING: duplicate object description of pyEQL.Solution.__init__.solvent, other instance in class_solution, use :no-index: for one of them

Because (I think) we have configured most of our unit tests to fail on warnings, these warnings cause our docs CI workflow to fail. We need to figure out how to address these warnings, or, modify the docs test to pass with them.