It was originally a community project, but was then abandoned for a while, and then donated to Rust proper. As such, it's always been sort of secondary. I plan on linking it more prominently once I've actually gone through it and cleaned it up to my own personal standards, for now I just keep CI green. As such, the text has gotten out of sync with the code in a number of places, and I don't want to mislead anyone.
Fair enough: it's definitely not a good idea to promote something half baked.
It actually took me a while to find the link there just now. I don't think "External Documentation" really imparts the right meaning. Perhaps something like "Other Guides and Resources" would make more sense?
We do link it from http://doc.rust-lang.org/ , but I guess that still says community...