Add /go/multi-window-api-ffi design doc redirect (#11762)

Add design doc for multi-window FFI API.

firebase.json

@@ -520,6 +520,7 @@
       { "source": "/go/multi-window-single-isolate", "destination": "https://docs.google.com/document/d/1Wdqs79TY3b1VkOZMDmt12psg5AhO-q9k5-Uv28Snr8M/edit?resourcekey=0-qvOf6jo8bR9W4ozLrbLMcQ", "type": 301 },
       { "source": "/go/multi-window-status", "destination": "https://docs.google.com/document/d/13E27tD8_9f6lDgwg3MpGNTV8XIRCZH3ByI-t9kI9IUM/edit?usp=sharing", "type": 301 },
       { "source": "/go/multi-window-api", "destination": "https://docs.google.com/document/d/1eQG-IS7r4_S9_h50MY_hSGwUVtgSTRSLzu7MLzPNd2Y/edit", "type": 301 },
+      { "source": "/go/multi-window-api-ffi", "destination": "https://docs.google.com/document/d/1UEaxd_aIlsPMzkfB6oyeYaKzwHadrWGyMzOS33oSbK4/edit", "type": 301 },      
       { "source": "/go/multiple-applications", "destination": "https://docs.google.com/document/d/1alrr7qimNDazmQx5D80BSHh62V2OINO4UQ5N3HcPnmo/edit?resourcekey=0-wPs1Go9AiKR-yG0-xStuKw", "type": 301 },
       { "source": "/go/multiple-engines", "destination": "https://docs.google.com/document/d/1NwiZPWHd1te46eP2GWwIezDV9CdMQkODAMuF5kWdtLw", "type": 301 },
       { "source": "/go/multiple-flutters", "destination": "https://docs.google.com/document/d/1fdKRufqUzQvERcqNIUSq-GdabXc4k8VIsClzRElJ6KY", "type": 301 },