Summarize the code architecture

[neiljp]

Description of documentation

This should summarize the structure of the application code, indicating broadly where certain elements are found, and flows of control occur.

Specifically not leaving much detail here, since this is TBD, self-assigned, and will evolve over time.

Mentioned as expansion of #27 at: https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/Developer.20Docs.20.23T27/near/1785157