Contract data type of nested and repeated data (`ARRAY` of `STRUCT`s) in BigQuery

### 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

Here's a couple possibilities:
- https://docs.getdbt.com/reference/resource-configs/contract
- https://docs.getdbt.com/reference/resource-configs/bigquery-configs

If the former, then the idea would be a section similar to https://docs.getdbt.com/reference/resource-configs/contract#size-precision-and-scale.

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

The contract `data_type` of a [nested and repeated fields in BigQuery](https://cloud.google.com/bigquery/docs/best-practices-performance-nested#using_nested_and_repeated_fields) should just be `array` like this:

```yaml
- name: repeated_struct.nested_repeated_struct
data_type: array
```

rather than have the full detailed data type like this:
```yaml
- name: repeated_struct.nested_repeated_struct
data_type: array>
```

### Additional information

See discussion here for background: https://github.com/dbt-labs/dbt-codegen/issues/190#issuecomment-2536512027

This is spiritually similar to a data type issue that came up with Snowflake as it related to [precision and scale of `NUMERIC`](https://docs.getdbt.com/reference/resource-configs/contract#size-precision-and-scale), but it is a distinct caveat.

Only tangentially related to https://github.com/dbt-labs/docs.getdbt.com/issues/3322.