Update OAuth in BigQuery Quickstart, Step 5

[runleonarun]

Contributions

• I have read the contribution docs, and understand what's expected of me.

Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/guides/bigquery?step=5

What part(s) of the page would you like to see updated?

• Customer feedback: https://docs.getdbt.com/quickstarts/bigquery?step=5 - does not mention that the OAuth config - https://docs.getdbt.com/docs/cloud/manage-access/set-up-bigquery-oauth - is necessary before developers can enter individual credentials. Also, unlike as with Snowflake, once account level credentials for GBQ are in place, individual developers can run queries, etc. This creates the illusion that setup is complete, which it is not. I would highly recommend mentioning the OAuth config step, its relevance, and linking to it in the quick start.

Additional information

Note this possibly related issue #4581