Hmm I thought they did too, and there were notes on it at some point. Maybe got lost with our recent docs platform migration, I'll follow up on that on our side.
But at the end of the day, you'll just need to mount a directory for your Retool containers, set that container directory path in the JDBC_DIRECTORY_PATH environment variable for your api and db-connector containers, and add your .jar files to that directory on the host for the containers to access them. Specifics on how you do that depends on how you're deployed, but in any case pretty analogous setup to the configuration needed for custom TLS certs.