We've kicked around the idea of having a "How do I..." section to Designer that would have much of the stuff you are asking about. What's good about making it part of the software, is that it is well bounded, tightly coupled, and could provide actual mechanisms/hooks to help with the question.
Not sure a "forum" is the proper mechanism for such a thing since anyone can post, and organizing it/maintaining it is beyond the scope of a public "forum".
Your idea is great, it's just that the actual implementation mechanism is the question (oh, and filling out the content, which is 99% of the work once we figure out the implementation

).