docs: add code snippets for session and IO public docs

[sycai]

Internal issue: 407575751

[sycai]

the presubmit failure is about "isdigit":

@pytest.mark.skipif(
        "dev" in pa.__version__,
        # [b/333484335](http://b/333484335) pyarrow is inconsistent on the behavior
        reason="pyarrow dev version is inconsistent on isdigit behavior.",
    )
    def test_isdigit(weird_strings, weird_strings_pd):
        pd_result = weird_strings_pd.str.isdigit()
        bf_result = weird_strings.str.isdigit().to_pandas()
    
>       pd.testing.assert_series_equal(
            bf_result,
            pd_result.astype(pd.BooleanDtype())
            # the dtype here is a case of intentional diversion from pandas
            # see [go/bigframes-dtypes](http://goto.google.com/bigframes-dtypes)
        )

This is not related to this change.

[chelsea-lin]

nit: to_pandas()

[sycai]

meticulous!

[snippet-bot]

Here is the summary of changes.

You are about to add 12 region tags.

• samples/snippets/sessions_and_io_test.py:20, tag bigquery_dataframes_create_and_use_session_instance
• samples/snippets/sessions_and_io_test.py:40, tag bigquery_dataframes_combine_data_from_multiple_sessions_raise_error
• samples/snippets/sessions_and_io_test.py:58, tag bigquery_dataframes_set_options_for_global_session
• samples/snippets/sessions_and_io_test.py:67, tag bigquery_dataframes_global_session_is_the_default_session
• samples/snippets/sessions_and_io_test.py:74, tag bigquery_dataframes_create_dataframe_from_py_and_np
• samples/snippets/sessions_and_io_test.py:94, tag bigquery_dataframes_create_dataframe_from_pandas
• samples/snippets/sessions_and_io_test.py:110, tag bigquery_dataframes_convert_bq_dataframe_to_pandas
• samples/snippets/sessions_and_io_test.py:123, tag bigquery_dataframes_to_pandas_dry_run
• samples/snippets/sessions_and_io_test.py:133, tag bigquery_dataframes_read_data_from_csv
• samples/snippets/sessions_and_io_test.py:141, tag bigquery_dataframes_read_data_from_bigquery_table
• samples/snippets/sessions_and_io_test.py:148, tag bigquery_dataframes_read_from_sql_query
• samples/snippets/sessions_and_io_test.py:163, tag bigquery_dataframes_dataframe_to_bigquery_table

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