chore: add onboarding doc with architecture diagrams

[Sajjad-Khoshdooni]

Background

New contributors currently have to piece together the architecture and service boundaries from multiple READMEs and code. This makes onboarding slower and increases the chance of misunderstanding the end-to-end order → auction → settlement flow.

Details

Create a new docs/ONBOARDING.md that:

• Explains the high-level architecture of the services in this repo
• Uses Mermaid diagrams to show:
  • System context (orderbook ↔ DB ↔ autopilot ↔ driver ↔ solvers ↔ chain)
  • The “happy path” sequence from order placement to on-chain settlement
  • Service responsibilities and key crates
• Includes a “Where to start reading code” section with practical entrypoints
• Links to existing docs (playground/README.md, database/README.md, docs/COW_ORDER_DEBUG_SKILL.md)

Also add a short link in README.md pointing to docs/ONBOARDING.md for discoverability.

Acceptance criteria

• docs/ONBOARDING.md exists and renders on GitHub (Mermaid diagrams)
• docs/ONBOARDING.md includes practical code entrypoints (orderbook/autopilot/driver/solvers/contracts/db)
• README.md links to docs/ONBOARDING.md
• Links to referenced docs resolve