Added documentation for LPLR model.

[jer2ig]

Documentation for logistic partially linear model on doubleml-for-py jh-logistic branch.
I have added the relevant files as far as I understand the structure of the repository, but it would be good if someone would check that nothing is missing.

[SvenKlaassen]

Personally, I would move the model directly after the PLR model before PLIV.

I think the main missing points are:

• An plot for the example gallery, which we can use for the thumbnail
• A description of the scores. It would be very helpful if you could include a short part on the estimation of the preliminary parameters similar to e.g. Quantile Models