Improved design doc

[andrewgordstewart]

Improve the design doc to clarify the design that we plan to implement.

This design doc is

1. a bit out of date. For instance, the ChallengeManager won't be constructed with the argument (commitment: StepCommitment[]), but instead will have a constructor signature more like (values: Bytes32[], disputedIndex: uint256)
2. consequentially, a bit overcomplicated. For instance, we now think that the first step invalidation is out of scope, and would be performed before deploying a ChallengeManager

OUT OF SCOPE: Describing the benefits and drawbacks relative to other designs. This will be done by @lalexgap

[andrewgordstewart]

I made a copy of Mike's version, and made the description more explicit.

The goal here is to describe DisputeGame in a way that facilitates the security analysis. This is important to Optimism; it is also important for us, as we plan to write a formal verification of the dispute protocol.

The explicit description also hopefully serves as an implementation guide.