Firstly, I’m sure paddle2see said before that it’s impossible to make a sub-forum in a forum. Secondly, The scratch team have considered your idea, and ejected it for the following reason:

9.7 Guide section on the forums
This proposed forum section would be a place for guides and tutorials that Scratchers could freely make with the purpose of helping others. However, the forums are generally a place for discussion; it is not a good place for topics with content that is not meant to be changed. A Scratch project or the Scratch Wiki are two good places to make tutorials.

This also applies for tutorials, as stated in the underlined bit