docs: add Flutter Driver example to MCP server page

[Ortes]

Summary

• Adds a third usage example to the MCP server docs page
• Shows how to connect to a running Flutter app and interact with its UI using Flutter Driver via the MCP server
• Links to the full setup guide in dart-lang/ai for details

Related

• Companion PR for dart-lang/ai: docs: add guide for connecting to a running Flutter app dart-lang/ai#414

Test plan

• Verify page builds and renders correctly
• Verify the :::note directive renders as expected
• Verify the link to the dart-lang/ai README section works

🤖 Generated with Claude Code

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