Added piecewise regression example

[speco29]

fixes #437
Adding piecewise regression examples in example docs

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[tomicapretto]

Hi @speco29, thanks again. In this case, the issue asks for an example notebook when one uses Bambi to perform a piecewise regression. You're pasting a generic example of piecewise regression with statsmodels.

[speco29]

hello @tomicapretto, thank you for your review.
Sorry, actually you are right, statsmodel doesn't belong here.
I'll look through it and will change it soon