Gitbook content update

I took a lot of mostly existing doc from VF, and some from NRP, and added to the gitbook content. This is very first cut, trying to get stuff out there so we can iterate. Some parts of it are rougher than others. In fact I need to even just test it in gitbook.

I also took recipe off of the UML diagram, but not Intent, although we need to figure that one out too. But it seems we should do that sooner.

I haven't messed with the classes and properties spec stuff, it is from the release doc from last year. (It isn't part of this PR, I just copied it and updated master earlier.) I'm hoping we can automate that and use the ttl file, as discussed somewhere. And maybe put it all back into two files, classes and properties.

I actually think it might be easiest to just merge this, and then iterate on it, so we can see it in its gitbook form.