Hey!
Sorry for the late response, we've had some holidays around midsummer.
Unfortunately we don't have plans currently to support the Discord Social SDK, we have a ticket in the backlog but it's unlikely to happen next release (nor the one after). As you found out, the only documentation that exists is how to use an existing integration, but nothing on creating new integrations. Your best bet here is to take a look at how e.g. the facebook or google auths are implementation.
If you'd like more support on implementation, we can do that under any of our support contracts. If you'd like I can put you in contact with our business development people. (or if you already have contact info for Mika, you can reach out directly)