Add go link for multi-window-api (#11315)

_Description of what this PR is changing or adding, and why:_
This adds a go-link for the Multi Window API proposal.

_Issues fixed by this PR (if any):_

_PRs or commits this PR depends on (if any):_

## Presubmit checklist

- [ ] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

firebase.json

@@ -509,6 +509,7 @@
       { "source": "/go/multi-view-sync-over-async", "destination": "https://docs.google.com/document/d/14z-e-x6M3P53ZMD6qBh4UaJ3nJdG_VdMNqGKERv31hw/edit?usp=sharing", "type": 301 },
       { "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/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 },