the tenets of openness tend to bleed into one another and limiting the site will constrain its growth, as well as what @warren said. @massonpj also pointed out there are already a plethora of sites for coding, so the real question/thought to me is, what makes this site niche? why would someone come here, and how could they find benefit(s) here? lastly, how to create a meaningful ux via community that is strong enough to drive/build itself.
having a defined scope/focus/etc., is extremely beneficial for giving purpose and guidance to users, as well as builds the basics for your generic "intro to open source se" guide.
i use opendata se daily, where this question rises daily, in one form or another. aside from a definition for guidance, its critical to have user leaders that have read the guide and are leading by example. that sounds kind of obvious for community building, but its well worth repeating.
users that follow the guidelines when posting typically are not, but i'd say there's one or two sent to gis se, data science se, stackoverflow, etc., daily.
here leaders are key: you want welcoming/helpful comments to newcomers, not only to put them on the correct path, but also to make them feel welcome, and encourage them to come back.
open textbooks falls under this category; i like to think of it as open source/free software for content.
teachers, authors, publishers (some, this also crosses into open access ;) ) and the like are going to be branching into the world of openness in droves in the future, and this is a great place for them to come and engage/learn/give back.