Add docs for using `psycopg2` with `dbt-postgres`

What are you changing in this pull request and why?

We changed how dbt-postgres is installed with regards to psycopg2 versus psycopg2-binary. In prior versions you could configure it with an environment variable. In 1.8, that's not an option because we changed our build tooling to a more modern stack. We initially tried an OS-driven approach, but that had issues mainly due to CI workflows that normally run on linux images. So in 1.8.2 we reverted back to installing psycopg2-binary by default and prescribed the manual workaround of uninstalling psycopg2-binary and reinstalling psycopg2.

Checklist

[x] Review the Content style guide so my content adheres to these guidelines.
[x] For docs versioning, review how to version a whole page and version a block of content.
[x] Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

dbt-labs / docs.getdbt.com

Add docs for using `psycopg2` with `dbt-postgres` #5707

What are you changing in this pull request and why?

Checklist