Add missing `scikit-learn` plots to `README.md`

[drbenvincent]

• modify docs/source/notebooks/generate_plots.ipynb to also generate and save the images from the scikit-learn version of the experiments
• add missing example for ANCOVA into README.md
• add missing example for Interrupted Time Series into README.md

[anevolbap]

In docs/source/notebooks/ there is no scikit-learn version for ANCOVA or RK experiments, just ITS (SC and DiD which are already present in README.md).

Should those examples be created within the scope of this issue?

I started this PR tackling the first and third point.

[drbenvincent]

Thanks for this @anevolbap

So this issue is partly (but not fully) addressed by #307. I think we can create separate issues/PR's for new scikit-learn models (if they don't already exist).

I commented on the PR #307 I think that PR is actually ready to merge.

[anevolbap]

@drbenvincent, I'll try to do the new scikit-learn models in a separate PR as you suggest. Afterwards, I hope to tackle more challenging issues.