EDIT EDIT Apparently this feature isn't ready yet. If you look at the version on the shcema template I posted you can see it is version 0.0.1-preview. Why they made an announcement stating publication of solution templates is ready when it explicitly relies on a 0.0.1-preview item is beyond me. The word I'm getting from Microsoft is that the current process is basically manual while they get the bugs out (understandable). The marketplace team is effectively working 1 on 1 with developers to publish things as part of this process (which is rather awesome).
In Summary there is no complete definition yet. Microsoft's Azure marketing team basically announced a feature to be ready at least a year before their Azure engineering dept had a chance to make a useful prototype (which is atrocious). The reason the docs aren't there is because Microsoft has a current trend of not documenting anything until after it's been in the wild long enough for the early adopters to debug and blog about (document) it for them. There won't be an actual answer with the complete schema for probably another few months.
EDIT It appears this is extremely similar to what the OP already found. I need this for work, so I'll update this with better info as I find it.
The link is basically an example showing how to use the CreateUiDefinition.json file. The actual schema is on that page (for now).
The "code" feature isn't working for me at the moment. Here is a snip from the only place I can find anything on this topic that is relevant: Simple-Windows-VM Template
