Improve GCP Billing docs

[gpop63]

We've noticed a gap in our GCP Billing integration docs — specifically, there's no clear explanation or example configurations for parameters like dataset_id. This can make setup confusing for users.

It'd be helpful to include a simple section with examples in our docs. For instance, we could explain that the dataset_id is the ID of your BigQuery dataset where billing data is stored, and table_pattern defines which tables to pull from. This would make setting up the integration way easier for everyone.

cc @ishleenk17