docs: add Snowflake and BigQuery drivers to Python API reference

[henryharbeck]

What feature or improvement would you like to see?

The following could be done to improve discoverability of the Snowflake and BigQuery drivers

• add Snowflake driver to Python API reference
• add BigQuery driver to Python API reference
• add BigQuery driver under the "Drivers" section of the docs

I understand that the BigQuery driver is not quite yet complete (ref #1974, #2172, #2893), so unsure if docs are desired at this stage.