Implementation example in the documentation doesn't get fully simplified

[david-pl]

Currently, the example in the documentation show-casing the implementation of position and momentum operators produces equations that aren't fully simplified:

This is probably due to the Div type introduced in SymbolicUtils. We may need to call out to different simplification routines, e.g. simplify_fractions when simplifying in the meanfield function. I couldn't find any recommendation in the docs on how to best simplify fractions though.