Add real-data example for multi-period DiD with DoubleML

[PhilippBach]

Adding an additional example to the gallery.

Small changes in the did model description.

[PhilippBach]

Hi @SvenKlaassen

here's the PR adding the real-data demo notebook replicating the from the did package for R.

I think we'd have to check/discuss

• What's the best way to load the data. Currently the notebook uses pyreadr's download_file() and then read_r() command, which is actually downloading the data set from the did package. I'm not sure if this is really optimal.
• If we want to replicate the balance_e (see screenshot taken from did example notebook below) exercise from the did notebook. I think this would be a nice illustrate to show how users can modify the weights, but I think it also gets a bit tricky. What do you mean?

[SvenKlaassen]

@PhilippBach , I think for the documentation it might be fine to use pyreadr, as we can similarily use other datasets from R packages.

The case of the balanced version is nice but might be a more advanced example. We could create seperate notebooks with more detailed focus in the future.

[PhilippBach]

Thanks @SvenKlaassen , then I guess this PR is ready for review / to be merged afterwards :)

[SvenKlaassen]

@PhilippBach
I have added the notebook to the gallery and moved the old notebooks down to the sandbox
Further, a small example text in the model highlights the relation to the original paper.
now we can merge it if the dev check passes.