docs: Add in Transfer Manager chunked upload/download samples

[sydney-munro]

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Fixes # ☕️

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[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 2 region tags.

- [samples/snippets/src/main/java/com/example/storage/transfermanager/AllowDivideAndConquerDownload.java:20](https://github.com/googleapis/java-storage/blob/abff6def593ff56a75301f4edbbce9470a613c37/samples/snippets/src/main/java/com/example/storage/transfermanager/AllowDivideAndConquerDownload.java#L20), tag `storage_transfer_manager_download_chunks_concurrently` - [samples/snippets/src/main/java/com/example/storage/transfermanager/AllowParallelCompositeUpload.java:20](https://github.com/googleapis/java-storage/blob/abff6def593ff56a75301f4edbbce9470a613c37/samples/snippets/src/main/java/com/example/storage/transfermanager/AllowParallelCompositeUpload.java#L20), tag `storage_transfer_manager_upload_chunks_concurrently`

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