docs: add snippet for creating boosted tree model

[rey-esp]

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Fixes # 🦕

[snippet-bot[bot]]

Here is the summary of changes.

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- [samples/snippets/classification_boosted_tree_model_test.py:42](https://github.com/googleapis/python-bigquery-dataframes/blob/a8ac72d903ddc26ef854693dc8d9fda5e2390435/samples/snippets/classification_boosted_tree_model_test.py#L42), tag `bigquery_dataframes_bqml_boosted_tree_create`

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