I am not breaking any news by acknowledging that Slack is one of my favorite asynchronous communication tools to get work done as a software developer. It is the place for my morning stand-ups, code review requests, and much more.
But why stop here?
Why not use Slack as your go-to channel to deliver training material during onboarding?
This is what Cassava does and much more. Take a look.
https://gocassava.com/blog/three-ways-cassava-streamline-software-developer-onboarding