docs: document AI Assistant setup and privacy - #2

Summary

• provide an administrator-first installation and OpenRouter configuration walkthrough
• document exact app, PHP, and Module API compatibility
• explain API-key funding, data sent to OpenRouter/model providers, permissions, and uninstall data behavior
• add a contribution guide that matches CI and the signed module release process

Validation

• PHP lint and 30 PHPUnit tests
• module and package validators
• pnpm lint, TypeScript check, build, and no dist drift
• Markdown and external-link checks