For the doc, I don't know if it lacks a lot of things, but I've found it quite good, and I've found only one little minor error in it.
But I think the idea of migrate it to a wiki is a good idea. wiki things are known to work!
For the demos, another kind of demos that I think that could be helpful would be test demos that test every feature of chipmunk, it would help to check everything works as it should, maybe for people that would try to hack chipmunk, and also it would help in my case when writing a binding to check that all the features work as expected through the wrapper.
For tutorials IMO I find it a very good idea to make a mix between demo and tutorial, as a demo with a lot of comments, as it is the case for moon-buggy. Because often when we read a tutorial we are willing to see it work to understand better the explanations, and it is quite boring to do copy/past to test the code, that is often not complete and doesn't work.
For the interactive demos, it would indeed help because it is not so easy when one is not used to physics, to figure what is the best way to model different things.
As you say that people are disappointed not having some interactive demos, but there is the Moon-Buggy that I found very nice ! Other like this would be wellcome. The only critic I can do about it is that there is no Makefile