docs: add snippet for predicting classifications using a boosted tree model

[rey-esp]

… model

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Fixes #<issue_number_goes_here> 🦕

[tswast]

Love it!

[tswast]

We'll need to wait for #1142 before we can merge this because of the tests, which need the model to be created via Python.

I don't think you will need to make any changes to this code sample besides resolving the merge conflicts to put this sample after the create model one, though.

[tswast]

Best not to rename these, though in this case it should be OK since we haven't added the samples to cloud.google.com/bigquery/docs yet

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/classification_boosted_tree_model_test.py:65, tag bigquery_dataframes_bqml_boosted_tree_evaluate
• samples/snippets/classification_boosted_tree_model_test.py:86, tag bigquery_dataframes_bqml_boosted_tree_predict

You are about to delete 1 region tag.

• samples/snippets/classification_boosted_tree_model_test.py:65, tag bigquery_dataframes_bqml_boosted_tree_explain

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