Research pages about rules

The problem

Some of the research pages will investigate the properties of a particular rule. We need a special directory for such pages together with an index where the rules will be arranged by categories, etc..

Possible solution

Create Research/Rules directory with a README.md file in it describing how the rules directory works, and how to create new pages in a consistent way.

Alternative solutions

It would certainly be nice to have a (more stable) version of the rule registry, but that's a lot of work, and I don't think this should be the primary focus. Instead, we should focus on manually explaining why a particular rule is interesting.