[Docs] More info on the server side of Keybase

Until I signed up and started using it I didn't really understand what sort of stuff was stored server side and what kind of architecture the server side of the service had. I think I have a vague idea now, but I would love to see one paragraph devoted to the topic in the main page's description of what Keybase is.

The section in the detailed tracking doc that asserts the client doesn't trust the messages it gets back from the server is neat - I'd like to see that highlighted on the front page.

Since the Keybase paradigm is fundamentally about peer verification, I'd love to see some comment on what would happen if the Keybase servers went away and what it would take to spin up mirror sites or alternative server side implementations that would protect against handing the Keybase servers full responsibility for maintaining the verification mesh.

Start with the main page's description of what Keybase is and the detailed tracking doc section about client trust in server messages. Confirm the server-side architecture and the requested discussion of server loss, mirrors, and alternative implementations with project context. Done means the main page includes an accurate, concise paragraph covering those points.

Written by the indexing model from the issue text.