Unfortunately we don't have any documentation yet for the beta, but I can give you a brief object overview here. Note that names could change before final release.
ToolWindowContainer is the control that represents one or more ToolWindows tab docked together. You can place a ToolWindowContainer as a child of DockSite.AutoHideLeftItems to make it an auto-hidden tool window group.
To make hierarchies of tool windows, you use SplitContainer as a child of DockSite. Then you can put other SplitContainers in that SplitContainer. It has an Orientation property. You can put ToolWindowContainers in as children of the SplitContainers.
Finally, when you want to have an MDI area or even an area that just hosts some other arbitrary content, put an MdiHost control in the SplitContainer. It is a ContentControl and you can put anything you wish in there. Note that this may eventually be renamed to a Workspace or a new base class introduced as Workspace.
Within MdiHost, we currently only have tabbed MDI but will likely add other types of MDI in the future. To do tabbed MDI, add a TabbedMdiRootContainer and then within that, one or more TabbedMdiContainers. Each one of those can contain multiple DocumentWindow classes.
As mentioned in the beta notes, tool windows aren't allowed in the document area yet however that will be resolved before final release.
Hope this helps get you going!