So, your question is a very interesting one. Many, many times , we see posts here on meta asking "Where can I ask XXX". At a glance asking here seems to be (currently) the best option to gauge (meta) people's opinions on where to post. Sometimes it is a clear cut case and everyone can agree that the OP should post their question on... programmers or... game dev or what ever. In my short time here on meta I have noticed that more often than not peoples opinions on where to post differs. One might say "don't post this on SO. Try programmers.SE" but the next comment is "That would be closed as off-topic there" and on and on and on.
The resounding (best advice) that I've seen given with regard to "where to post my question" lies deeper than simply reading the FAQ for that specific site. It involves taking a look at some of the questions posted on that site and comparing its topics to your own question. This is the best way to know if you are posting in the correct place.
All of the SE sites are community run with regard to deciding what is on/off-topic. This manifests itself (amongst other things) in the form of close votes for any >3K users. All you need is five people within the community to feel that it is an off topic post and poof!
[closed]. If the OP is lucky if they get a comment pointing them in the right direction but that is not always the case (an issue for a different post). Because of this design the type of accepted posts on any site can sway to any direction depending on the currently active users.
Stack Overflow is extremely strict with regard to what questions are allowed - one of the reasons is the sheer amount of posts that are made. On a smaller SE site, with less traffic, users might be more lenient and flexible with regard to what is allowed.
Regarding your question/request for a centralized FAQ area.
I think this might be quite a large and probably not too efficient way to convey where to post what. It would have to contain sample questions as references and IMO it would simply be too big to use it as a reference (especially for new users who just want to ask their question already! ;)
The idea is a good one - users should have a place to know and learn where to ask what questions - the solution for that is simply as I said above - take a look at some of the posts on a few sites - you'll see exactly
- what is allowed
- what is tolerated
- what is off-topic