Okay, sorry for taking forever here folks, this has just been hard. My initial post said:
The reason that this hasn't happened is that the docs team has been effectively dead for a long time, and we are finally actually making that official: https://github.com/rust-lang/blog.rust-lang.org/pull/536
Additionally, @petrochenkov, @mark-i-m, and @Centril were all right about different things in different ways: there's no point in starting a huge project super late in the year, both due to roadmap issues as well as that work always slows down around the holidays. There weren't a lot of folks that were actually going to do this work, and there are folks who have been putting in great work on the reference without this being some official big project. Finally, there is just some foundational stuff that's not quite nailed down yet, and so isn't really ready to be documented yet.
If you're interested in contributing to the reference, please do: it still needs help, and there is still a lot of work to do. And folks working on it. Just this plan hasn't worked out so far.