buckets: document CopyFile operation for storage buckets

[mishig25]

Summary

• Document the copy files operation for storage buckets
• Show CLI (hf buckets cp) and Python (api.copy_files) usage
• Note that only Xet-tracked files can be copied server-side
• Update the "Data processing pipelines" use case to reference the new copy capability

🤖 Generated with Claude Code

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Note

Low Risk
Low risk documentation-only change that adds guidance for hf buckets cp/HfApi.copy_files and updates a use-case reference; no runtime or API behavior is modified.

Overview
Adds documentation for server-side copying of Xet-tracked files from Hub repos or other buckets into a destination bucket, including CLI (hf buckets cp) and Python (HfApi.copy_files) examples and a note about non-Xet files being re-uploaded.

Updates the Data processing pipelines section to reference this new copy capability instead of stating it’s unavailable.

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[HuggingFaceDocBuilderDev]

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