docs: add onboarding diagrams and entrypoints

[jmg-duarte]

Description

Addresses review feedback on #4296.

New contributors need a single place to see how orderbook, autopilot, driver, solvers, Postgres, and the chain fit together, plus where to open the codebase for each area. This PR adds docs/ONBOARDING.md with Mermaid diagrams and practical code entrypoints, and links it from README.md.

Changes

• Add docs/ONBOARDING.md: system context diagram, order→auction→settle sequence diagram, service responsibilities diagram, DB mental model diagram
• Add "Where to start reading code" with entrypoints pointing to run.rs for each service (orderbook, autopilot, driver, solvers, contracts, database)
• Add solver types section explaining colocated vs non-colocated distinction
• Add local development quick-start (playground with ETH_RPC_URL note, cargo check/nextest, formatting)
• Add debugging section linking to COW_ORDER_DEBUG_SKILL.md
• Update README.md with a link to docs/ONBOARDING.md

How to test

1. Open docs/ONBOARDING.md on GitHub and confirm all Mermaid blocks render (context flowchart, sequence diagram, responsibilities flowchart, DB flowchart)
2. Click links to playground/README.md, database/README.md, and COW_ORDER_DEBUG_SKILL.md and confirm they resolve
3. Open README.md and confirm the new onboarding link points to ./docs/ONBOARDING.md

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive onboarding guide and architecture diagrams for the CoW Protocol services. The review identified several critical logic and syntax errors in the Mermaid diagrams, specifically the use of non-standard 'smart' quotes that break rendering and incorrect loop boundaries in the sequence diagram that failed to capture the full auction lifecycle. Additionally, the system context diagram was updated to correctly reflect the auction distribution flow to colocated solvers.

[MartinquaXD]

Looks good overall. I imagine this should be helpful for new contributors / colleagues.

[tilacog]

Super nice diagrams!

[tilacog]

nit: I'd flip this arrow, since the events originate on-chain and are captured by the autopilot.

[jmg-duarte]

Here and in the other comment, I think the distinction will always be in the eye of the beholder, I understand both ways of thinking

Autopilot actively indexes events by querying the chain
Autopilot actively indexes events from the chain

I'll keep them as is