docs: Add FAQ section

[meichuanyi]

Adds a comprehensive FAQ section covering:

• What is Atlassian Rovo MCP Server
• Supported clients (OpenAI ChatGPT/Claude/GitHub Copilot/Gemini CLI/VS Code)
• Authentication options (OAuth 2.1 vs API Token)
• Admin approval requirements
• Permission management
• Local client setup
• MCP endpoint
• Skills usage
• Token optimization tips (AGENTS.md defaults)
• IP allowlisting troubleshooting
• Admin management controls
• Security measures
• MCP security risks and mitigation
• Help resources

The FAQ provides quick answers to common questions and links to relevant documentation sections for deeper understanding.

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