docs: document Vercel deep clone requirement for last update time

[SreehariU]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Documents the required VERCEL_DEEP_CLONE=true setting when using
showLastUpdateTime with Vercel deployments.

Vercel performs shallow Git clones by default, which can result in
missing or incorrect last update metadata when this option is enabled.

Test Plan

• Verified documentation renders correctly in local development
• Checked deploy preview

Test links

Deploy preview:
https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

Fixes #10031

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