docs: make README clearer and more OSS-style

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Summary

• lead with the product instead of internal architecture terms
• move quick start directly under the intro
• replace abstract wording with plain descriptions of control, simulation, replay, navigation, hardware, and safety
• keep the important safety boundary without repeating it throughout the page
• remove public README bookkeeping about draft PRs
• keep contributor and coding-agent setup, but shorten it

No runtime behavior changed.