Document target, modular tor architecture

We'll be working during sponsor31 to make sure that we are moving towards a nice modular architecture. We should document what we're actually trying to achieve, and what our target architecture is, so that we can tell people "do it like X, not necessarily like Tor does it now."

The official deliverable here is "Architectural documentation for how Tor modules work with one another, covering both the actuality and the refactored architecture". The "actuality" is under legacy/trac#29214 (moved).