Add an index table to the README.md

[JoshuaKGoldberg]

The README.md is really really sparse right now:

typescript-eslint Examples

Various examples of working with typescript-eslint. 📝
We may add to this over time, or we may get rid of it in favor of typescript-eslint.io.
We'll see.

Folks have to click into packages/ to see them, and even then each needs its own README.md read to learn more than just what's in the title. Let's add a nice summary table explaining what each is!