Confusion about Instances{peano}: document reasoning behind Instances0, Instances1, ..

I submitted my first, albeit small PR just now. One thing that confused me was the presence of traits like: XInstances0, XInstances1, XInstances2, etc. I understand the existence of XInstances but why are they spread over the naturals like that?

This gave me a little anxiety when I was submitting my first PR because I wanted to ensure I put my first instance in the right place. I have no idea where my instance resides in the peano hierarchy of random instances, nor, given my scala experience, could I infer why things were designed this way.

Suggestions:

• somewhere in documentation (or contributing guidelines) give historical reasoning for this design choice.
• in same documentation give simple criteria for how to determine which peano trait to place your instance.

I hope that's clear. Also, I'm so curious to find out the history behind this!

Start by locating the XInstances, XInstances0, XInstances1, and related Peano traits in the Cats repository, then review the documentation or contributing guidelines suggested in the issue. Research the historical reason for splitting instances across the hierarchy and define simple criteria for choosing a trait. Done means the reasoning and placement guidance are documented for newcomers.