Add go redirect for table-cell-span doc (#12602)

## Description of what this PR is changing or adding, and why:
Adds a new shortlink redirect for the Flutter design document describing
TableCell span support.

## Issues fixed by this PR (if any):


## PRs or commits this PR depends on (if any):
[flutter/flutter#177102](flutter/flutter#177102)

Author: hm21

firebase.json

@@ -663,6 +663,7 @@
       { "source": "/go/swift-package-manager-plugins-v3", "destination": "https://docs.google.com/document/d/1aoa-t6zZBc6TosCa8N41w8aA8O4Z5yqdV4DvSRjWC54/edit?usp=sharing", "type": 301 },
       { "source": "/go/synchronized-widgettester", "destination": "https://docs.google.com/document/d/1VumsuG6dEFUVpPQLqqKJnhI0CoIS9fCAMN-NFHIPmo0/edit", "type": 301 },
       { "source": "/go/system-mouse-cursor", "destination": "https://docs.google.com/document/d/1bJLRy6flZ0wDCbpl2QA8SURUWXIvRJKMRRemxlOo1cA/edit", "type": 301 },
+      { "source": "/go/table-cell-span", "destination": "https://docs.google.com/document/d/1OXTnWhI1bjZEfUsE3Cy8tm32ewMIwqKLXBKZPgTwGTM/edit?usp=sharing", "type": 301 },
       { "source": "/go/table-development", "destination": "https://docs.google.com/document/d/1fCE-zQNql0nnqJXhycpg902lhL7jrOOz-6sT8s8tv5c/edit?usp=sharing", "type": 301 },
       { "source": "/go/table-view", "destination": "https://docs.google.com/document/d/15ecTZE1g3WeswLGFWrnEgMP6SyL6jDRdxOgPsczOcV0/edit?usp=sharing&resourcekey=0-yNd_qFhiPjz6z2TgezWc0A", "type": 301 },
       { "source": "/go/table-view-merged-cells", "destination": "https://docs.google.com/document/d/1UekXjG_VKmWYbsxDEzMqTb7F-6oUr05v998n5IqtVWs/edit?usp=sharing", "type": 301 },