*Regression to the Mean* and *Euclidean distances in high dimensions*

*Regression to the mean* is so important, and I may not be doing it justice in my vignettes. (Also, it is great example of another topic that belongs in `ranking`).

I've also added *Euclidean distances in high dimensions*. Students and practitioners are often taught to use cosine similarity in high-dimension and it's rarely explained why, so I thought I add the problem with using more traditional metrics.