Chore: use engine_adapter.execute for statements in python model docs

[georgesittas]

Using fetchdf() for statements may result in errors, e.g.:

File ".../site-packages/pandas/io/sql.py", line 2729, in read_query
        columns = [col_desc[0] for col_desc in cursor.description]
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    
          TypeError: 'NoneType' object is not iterable