Split plugin chapters out of user guide

The gradle userguide is pretty big, almost big enough that it is hard to use. Just the table of contents is big enough that it often takes quite a bit of scrolling to find the chapter you are interested in. It might make the user guide more usable to split it into two, a general user guide with chapters 1-20, 50, 51 and 54-65 and a official plugin guide with chapters 21-49, 52, and 53. Just a thought.