Does anyone know of a good tutorial or application for asking and answering questions for Ruby on Rails, much like this site (although it probably will be for a very different purpose in the end)?
If you're serious about building this application, I can only suggest to just learn Ruby on Rails and build it yourself, instead of looking for a tutorial. There's plenty of great learning material, and assuming you've got a decent understanding of programming in Ruby, Rails is incredibly easy to get started with. As far as books go, Agile Web Development with Rails definitely gets my recommendation.
Either way, the Rails Guides are probably a good first read, It'll help you understand the basics of MVC and how to build a simple Rails app, so you can jump in and start building your own application. If you run into any problems down the road, post your questions here and I'm sure you'll get the answer you're looking for.
Even if tutorials existed for building every kind of application, knowing what's going on and how to architect your application around your specific needs is really important.