Add example to train a linear model for molecular polarizability

[ppegolo]

Draft for a recipe to train a linear ridge regression model with scikit-learn and featomic descriptors.
The model is then wrapped in a metatomic model and evaluated on ASE Atoms.

[ppegolo]

@ceriottm were you thinking of something like this?

[github-actions]

Here is the build version of the cookbook in this pull request: documentation.zip, you can view it locally by unzipping documentation.zip and open the index.html with your favorite browser.