docs: Add FAQ section for common questions

[meichuanyi]

Summary

Add comprehensive FAQ section to help users understand OpenSandbox quickly.

Changes

• Add FAQ section covering:
  • What is OpenSandbox
  • Comparison with other sandbox solutions (E2B/Daytona/Modal)
  • Key features table
  • Available SDKs (5 languages)
  • CLI usage examples
  • MCP server setup
  • Use cases (Coding Agents/Browser Automation/ML Training)
  • Secure container runtimes (gVisor/Kata/Firecracker)
  • Kubernetes deployment
  • Project structure
  • License and help resources

Checklist

• Documentation update only (no code changes)
• Content is accurate and helpful
• Follows existing README formatting style

[CLAassistant]

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[github-actions]

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Changed directories: README.md.

💡 Tip: Use feature for new functionality or improvements, bug for fixes.

cc @meichuanyi