[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.

Title:
GH-<ISSUE_NUMBER>: [<COMPONENT>] SUMMARY

PR description should include the standard sections:

• Rationale for this change
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[uros-b]

Other than @Reranko05's note above, code changes look good to me!

[thisisnic]

Thanks for looking at this. This change doesn't feel like it makes sense here as we end up jumping between the NYC taxi dataset and another hypothetical one. It's probably worth us thinking about where it'll make more sense for the reader. The examples for the open_dataset() function might be a better location than this vignette.

Removing approval as changes are required