setup docs for %%bigquery magics

Sub-tasks:

[ ] Let's get nox -r -s docs and nox -r -s docfx passing.
[ ] API reference docs render at least this much content: https://cloud.google.com/python/docs/reference/bigquery/latest/magics
[ ] Ideally we also document every supported parameter. (e.g. "params" and "location")
[ ] Deploy the docs as part of the release (either to Cloud RAD cloud.google.com/python or Sphinx googleapis.dev)
[ ] Update code samples at https://cloud.google.com/bigquery/docs/visualize-jupyter

googleapis / python-bigquery-magics