Document setup steps for databases for CI tests

### Describe the feature

The CI tests for this dbt package run against the following databases:
- Postgres
- BigQuery
- Redshift
- Snowflake

We want to be able to set these up from scratch and know how to configure them for use in CI. This includes things like necessary permissions, expected settings that differ from the default, and how to accomplish authentication/authorization via environment variables.

So we should create documentation that outlines all of the necessary steps to set these up from scratch.

### Who will this benefit?
What kind of use case will this feature be useful for? Please be specific and provide examples, this will help us prioritize properly.

This will directly benefit the maintainers of this package by empowering setup from scratch. It could also indirectly benefit maintainers of other dbt Labs packages for the same reason.

### Anything else?

Instigated by:
- https://github.com/dbt-labs/dbt-codegen/pull/203
- https://github.com/dbt-labs/dbt-codegen/pull/204
- https://github.com/dbt-labs/dbt-codegen/pull/208

Start by reviewing the CI test setup for Postgres, BigQuery, Redshift, and Snowflake, then read the referenced pull requests 203, 204, and 208 for existing setup details. Document permissions, non-default settings, and environment-variable authentication and authorization for each database so maintainers can reproduce the CI setup from scratch.

Written by the indexing model from the issue text.