Add `pandas` to intersphinx links.

[tswast]

https://googleapis.dev/python/bigquery/latest/generated/google.cloud.bigquery.table.RowIterator.html#google.cloud.bigquery.table.RowIterator.to_dataframe currently does not link to the pandas documentation on DataFrame.

Adding pandas to the intersphinx section should address this.

Note: there is a template variable for this in owlbot config:

python-bigquery/owlbot.py

Line 95 in c44d45b

templated_files = common.py_library(