docs: add snippet for evaluating a boosted tree model

[rey-esp]

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For inclusion in the Create a boosted tree classification model in BigQuery ML tutorial 🦕

[snippet-bot[bot]]

Here is the summary of changes.

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- [samples/snippets/classification_boosted_tree_model_test.py:65](https://github.com/googleapis/python-bigquery-dataframes/blob/6a70279f51db5be9bef900eb8c1596042e8a3124/samples/snippets/classification_boosted_tree_model_test.py#L65), tag `bigquery_dataframes_bqml_boosted_tree_explain`

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