Add go link for multi-window-api

[Saviq]

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.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

lgtm

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 9a0f67d):

https://flutter-docs-prod--pr11315-patch-1-8vs7i2gi.web.app