Nested and repeated BigQuery columns using `RECORD` / `STRUCT`

### Contributions

- [X] I have read the contribution docs, and understand what's expected of me.

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

_Maybe_ this page?
https://docs.getdbt.com/reference/resource-configs/bigquery-configs

Fine with me if it's a different page as long as it comes up in Google or Algolia searches.

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

BigQuery allows [nested and repeated columns in table schemas](https://cloud.google.com/bigquery/docs/nested-repeated).

I couldn't find the docs on this related feature in dbt-bigquery: https://github.com/dbt-labs/dbt-core/issues/2549

I tried searches like the following in Google and Algolia search box in docs.getdbt.com:
- "dbt bigquery struct nested"
- "dbt bigquery nested record"

### Additional information

Instigating context was triaging this bug report: https://github.com/dbt-labs/dbt-bigquery/issues/687