Add redirect for impeller backend desktop

[sero583]

_Description of what this PR is changing or adding, and why: Adds a redirect for a design document.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any): flutter/flutter#183382

Presubmit checklist

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  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request adds a new redirect for /go/impeller-backend-desktop to a Google Doc. The change is straightforward and correctly placed alphabetically within the firebase.json configuration. I have one minor suggestion to improve the destination URL for consistency and to prevent a potential extra redirect.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit f73b3c8):

https://flutter-docs-prod--pr13164-patch-1-8iapdoeh.web.app

[sfshaza2]

lgtm

[sero583]

Thank you! :)