Add /go/onmaploaded design doc redirect

[hearsilent]

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

Adds a flutter.dev/go redirect for the public onMapLoaded Cross-Platform Semantics.

This is needed so the design doc follows the Flutter design doc process and can be referenced from the tracking issue and review discussions.

Issues fixed by this PR (if any):

flutter/flutter#99610

PRs or commits this PR depends on (if any):

flutter/packages#10714

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• 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 pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request adds a new redirect from /go/onmaploaded to a Google Docs design document. The change is straightforward, adding a new entry to the redirects array in firebase.json. The new entry is correctly placed to maintain the alphabetical order of the source paths, and its format is consistent with existing redirects in the file. I have reviewed the change and found no issues.

[parlough]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 87ca5eb):

https://flutter-docs-prod--pr13200-patch-1-rd7afu52.web.app