Documentation Visual for `hill_saturation` function

[williambdean]

The adstock and saturation tranformations in the transformers module all display example
parameter values in order to display the shape of function. However, hill is not implemented or not displayed.

This should be added to the docstring of the hill_saturation function

The requirements here:

• match the style of the other functions in the module
• display handful of different parameter values to show the shape of the function

The most similar function would be the michaelis_menten since alpha and sigma are both scaling parameters.

[PatrickRobotham]

The issue is caused by the plotting code for hill_saturation having a bug.
I've submitted a PR here #857