Update docs to add info about using FVM with Dart MCP

[Harishwarrior]

We use AI every day to develop Flutter apps. But I noticed the Dart MCP always fails to use the fvm. I have updated the MCP config from the official docs. It would be great if we could add this to the docs

Source: dart-lang/ai#319 (comment)

{
    "mcpServers": {
        "dart": {
            "command": "dart",
            "args": [
                "mcp-server",
                "--flutter-sdk",
                ".fvm/flutter_sdk"
            ]
        }
    }
}