doc: add abbreviated upspin overview to signup docs

[adg]

On the Signup page, the first thing a new Upspin user sees, we provide a link to the Overview before diving into a discussion of the tools. The Overview document itself is long and certainly not required reading to get started with Upspin.

We should add a short overview of the system to the Signup page, perhaps including the diagrams on the Architecture page, so that a first-time user knows what to expect as a result of signing up.