Add /docs dir(s?), begin writing (light) documentation

Probably a root-level `/doc`s dir for cross-cutting documentation and architecture diagrams, maybe `/{service}/docs` dirs for service specific docs as well, although to start the service level READMEs could be sufficient.

Preferably we just document architecture, APIs, and development requirements. Generally, the code should be "self-documenting".

We should start maintaining architecture diagrams in the root docs dir, probably made with d2.