docs: add snippets for performance optimization doc

[sycai]

For #411054336 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• samples/snippets/performance_optimizations_test.py:17, tag bigquery_bigframes_use_peek_to_preview_data
• samples/snippets/performance_optimizations_test.py:34, tag bigquery_bigframes_use_cache_after_expensive_operations
• samples/snippets/performance_optimizations_test.py:47, tag bigquery_bigframes_enable_deferred_repr_for_debugging

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