Improve math rendering of models in the docs

[drbenvincent]

#232 introduced better docstrings which include mathematical specifications of the models, like this:

In an ideal world we could pimp these maths blocks, e.g.

• get the equations aligned on the $\sim$ or $=$
• use greek symbols such as $\beta, \sigma, \mu$ etc
• get operators to be formatted properly, e.g. \mathrm{Normal}

We could optionally add in the model math for the WeightedProportion model:

CausalPy/causalpy/skl_models.py

Line 15 in 234a0cd

class WeightedProportion(LinearModel, RegressorMixin):