Add design doc redirect for ffi-in-embedders (#11824)

Will be live at https://flutter.dev/go/ffi-in-embedders

Author: knopp

firebase.json

@@ -379,6 +379,7 @@
       { "source": "/go/faster-cocoon", "destination": "https://docs.google.com/document/d/1NXP0_9TJ1qz674iYyOGVLEHWVpmbXr5yjhBrdoU_x5g/edit", "type": 301 },
       { "source": "/go/federated-platforms", "destination": "https://docs.google.com/document/d/1z_4Z5JMTbk5c4FpayVAb-PTz2wLP-aSCirbieavT3Ws/edit?usp=sharing", "type": 301 },
       { "source": "/go/federated-plugins", "destination": "https://docs.google.com/document/d/1LD7QjmzJZLCopUrFAAE98wOUQpjmguyGTN2wd_89Srs/edit", "type": 301 },
+      { "source": "/go/ffi-in-embedders", "destination": "https://docs.google.com/document/d/11siUQs1Fn7G1Baue6YYrJHT0ieNiC-956yVv4gqAhr0/edit?tab=t.0", "type": 301 },
       { "source": "/go/ffi-plugins", "destination": "https://docs.google.com/document/d/1vMw-W6L-r-uL1SUgzDh6J5rmtqum7SZdO9khWACrZJI/edit?resourcekey=0-vvjEK88N-pApyKTPWJ2IZQ", "type": 301 },
       { "source": "/go/file-system-access-from-devtools", "destination": "https://docs.google.com/document/d/1BNgZka2N9HYL11A4IdR8FqsSW1VMr9Vo7CSrjY9rcic/edit?usp=sharing&resourcekey=0-H1f6lS5KjytrUCWc1vcS_w", "type": 301 },
       { "source": "/go/flame-dartpad", "destination": "https://dartpad.dev/?id=3e52ca7b51ba15f989ad880b8b3314a2", "type": 301 },