Document high-level MetaMask architecture (from a package/controller perspective)

Someone new to MetaMask might look at all of the repos that we have and (after they get over their initial shock) ask, how do all of these pieces fit together?

There are a few ways we could answer that question, but to start, we could create an architecture graph that includes the extension, mobile app, and packages that are used along the way (including core controllers, but also libraries like `eth-sig-util`). To provide another view, we could have a list below the graph which summarizes all of the packages with a short summary of the role that they play. Then we put both of these things in a new document.

Start by mapping the extension, mobile app, core controllers, and packages mentioned in the issue, including eth-sig-util, from the package/controller perspective. Decide how those pieces should appear in an architecture graph and companion package list. Done means a new document contains both views and gives each package a short role summary.

Written by the indexing model from the issue text.