feat: use bigquery_magics package for the %%bigquery magic

[aliciawilliams]

Requesting update to magics.rst file that populates IPython Magics for BigQuery documentation to reference bigquery_magics package (rather than google.cloud.bigquery).

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

[Linchin]

closed by #2125