Add design doc for merged cells (flutter#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

- [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.

Author: Piinks