docs: Add in Transfer Manager chunked upload/download samples

[sydney-munro]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the samples format.

[snippet-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, tag storage_transfer_manager_download_chunks_concurrently
• samples/snippets/src/main/java/com/example/storage/transfermanager/AllowParallelCompositeUpload.java:20, tag storage_transfer_manager_upload_chunks_concurrently

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment