We hope to update the documentation over time, to get started though, I would take a look at our Help Center as a lot of the RapidWeaver 7 documentation is still applicable.
https://help.realmacsoftware.com/
We’re also going to be doing more free tutorials in the next few weeks and months, so keep an eye on this page: https://www.realmacsoftware.com/rapidweaver/tutorials/ (there’s already plenty of great videos for 8 on there).
Hope that helps.