Add design doc for merged cells (#10086)

_Description of what this PR is changing or adding, and why:_

Design doc for flutter/packages#5917

_Issues fixed by this PR (if any):_

Related to flutter/flutter#131224

## Presubmit checklist

- [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

@@ -541,6 +541,7 @@
       { "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-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 },
       { "source": "/go/tabs-and-text-fields", "destination": "https://docs.google.com/document/d/1aHucsI0NWGWu2Dm_XFsBLxiTgJRM8h2XBB7PVAnVxlU/edit?usp=sharing&resourcekey=0-zLbXFlP_A2e_Yoi43vdiiw", "type": 301 },
       { "source": "/go/take-flutter-home", "destination": "https://github.com/flutter/put-flutter-to-work", "type": 301 },
       { "source": "/go/table-development", "destination": "https://docs.google.com/document/d/1fCE-zQNql0nnqJXhycpg902lhL7jrOOz-6sT8s8tv5c/edit?usp=sharing", "type": 301 },