Add redirect for overlay-semantics documentation

[chunhtai]

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

Issues fixed by this PR (if any):

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

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[gemini-code-assist]

Code Review

This pull request adds a new 301 redirect for '/go/overlay-semantics' to a Google Document in the firebase.json configuration. I have no feedback to provide.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 72bedeb):

https://flutter-docs-prod--pr13374-chunhtai-patch-1-qwkispnz.web.app

[sfshaza2]

lgtm