docs: add FAQ section for common questions

Summary

This PR adds a comprehensive FAQ section to the README, covering:

• General: Platform overview, differences from traditional pentest tools
• Installation & Setup: Python version, installation steps, API keys
• LLM Providers: Supported models, local model usage
• Modes & Usage: Assist/Agent/Crew/Interact modes, playbook usage
• MCP & Tools: MCP server configuration, spawn_mcp_agent, child agents
• Docker: Docker mode benefits and usage
• Troubleshooting: Common issues and solutions
• Legal & Ethics: Authorization requirements

Why

Users often have common questions about:

1. How PentestAgent differs from existing tools
2. Which LLM providers are supported
3. How to use different modes effectively
4. MCP tool configuration and child agent spawning
5. Common troubleshooting scenarios

This FAQ addresses these questions directly in the README, improving the onboarding experience.

Checklist

• Documentation added/updated
• No code changes (documentation only)