Add table to documentation mapping mypy/pyright rules to ty rules?

[AlexWaygood]

For some of our rules there isn't a 1:1 mapping, but for a lot of rules there is. This could be really helpful for users migrating from other type checkers.

I'm not sure we should do it now, though, because our ruleset is still in quite a high level of flux. We should probably wait until we're closer to stable before attempting something like this.