[R][Docs] Document preserving leading zeros in partitions

[tomatotomata]

Summary\n\n- document why inferred partition types can strip meaningful leading zeros\n- show the explicit hive_partition(product_id = string()) pattern\n\nThis addresses the documentation portion of #39660 and keeps the change to the R dataset vignette. I verified the patch with git diff --check; the added R example is syntactically valid. The full vignette render is not available in this checkout because the Arrow R package and its vignette dependencies are not installed.

[github-actions]

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[Reranko05]

Please update the PR title and description to follow Arrow's required format.

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