docs: clarify install and plugin settings

[riatzukiza]

Summary

• surface minimal provider config first with single-model quick start and relocate plugin-level settings near install
• remove non-functional built-in command description and deduplicate plugin settings references
• update docs/config anchors plus spec notes to match new headings and flow

Testing

• Not run (docs-only changes)

[coderabbitai]

Important

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch docs/readme-dedupe

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