feat: use bigquery_magics package for the %%bigquery magic

Requesting update to [magics.rst file](https://github.com/googleapis/python-bigquery/blob/main/docs/magics.rst) that populates [IPython Magics for BigQuery](https://cloud.google.com/python/docs/reference/bigquery/latest/magics) documentation to reference bigquery_magics package (rather than google.cloud.bigquery).

Similar PR: https://github.com/googleapis/python-bigquery/pull/1965/files 