No way. First, it has to be hosted in house. Second, that's basically the same markdown + pandoc I use for free already. Except it's even less flexible, and costs money. If API documentation would be just an ordinary book, I wouldn't have the urge to change anything, as pandoc + custom CSS actually does fine. Generating HTML + TOC from a bunch of .md files isn't really a problem, you know, it just isn't what makes writing API docs painful. I have a couple of sentences to say about every RPC call, maybe. It's actually mostly about providing huge (so, I guess foldable) params/result examples and making it easier to change the docs when the code changes. See Slate for example. Or even readthedocs. Both are not even remotely perfect, but are wa-ay more suitable for writing docs than gitbook.
