I forgot to clarify that the training class @polyester and others and I gave is something that Plone has offered for many years, but we didn't have formal training materials for it because the way the class went depended on the mix of people who showed up, and they often had very specific things they wanted to learn.
That was fine because we had the Working with Content documentation for Classic, but once we were mostly switched to Volto it was no longer as useful so we needed an updated training guide, which we now have (but it still needs a lot added).
We do have the start of a Volto user guide in the documentation.