docs: add sample for dataset copy

[tswast]

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Towards internal issue 174766824
Fixes #70 🦕

[snippet-bot]

Here is the summary of changes.

You added 2 region tags.

• samples/snippets/copy_dataset.py:17, tag bigquerydatatransfer_copy_dataset
• samples/snippets/copy_dataset.py:37, tag bigquerydatatransfer_copy_dataset

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[tswast]

Sample published here: https://cloud.google.com/bigquery/docs/copying-datasets?hl=en#python