docs: add FAQ section for common questions

[meichuanyi]

Summary

Add a comprehensive FAQ section to the README covering common questions about Docker Agent.

Changes

• Added FAQ section with 15 Q&A items covering:
  • General: What is Docker Agent, differences from other frameworks, Docker version requirements
  • Installation & Configuration: Installation methods, AI provider support, local models
  • Agent Configuration: YAML agent definitions, multi-agent architecture, RAG
  • Tools & MCP: Available tools, MCP server integration
  • Deployment & Sharing: OCI registry push/pull, agent catalog
  • Troubleshooting: Authentication errors, MCP connection issues, timeouts, TUI display

Motivation

New users frequently ask about AI provider configuration, MCP tool setup, and agent sharing. This FAQ provides quick answers to the most common questions, reducing friction for onboarding and complementing the existing documentation site.

Checklist

• Documentation follows project style (English, consistent with existing README)
• Content is specific to Docker Agent features (YAML config, OCI registry, MCP, Docker Model Runner)
• No code changes, documentation only