Write ARCHITECTURE.md

We'd like to add a top level architecture document, to act as a quick explainer for new contributors to understand the decisions around the project layout, what belongs where, and where proper boundaries maintained between modules. We should not shy away from describing where "bodies are buried" either.

Inspiration:

https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html

some areas that jump to mind for me (@ramin):

- what warrant top level modules (eg: `das`, `core`, `header`)
- `nodebuilder`
- `share` and its constituent modules with a brief description of each
- `state`
- p2p high level design
- rpc | cmd | cli | gateway