Documentation is broad, and it is a general reference. You aren't documenting a specific problem you're facing, you're helping others deal with an entire class of problems by documenting.
http://docs.plone.org has the same goal. Does it make sense to have this on SO? Of course it's easier to contribute on SO, but maintaining the same content at two places also does not make sense.
Leave documentation and the process where it is: on Github using Sphinx....you don't want to change the workflow for maintaining docs every other years just because of some new car passes by.
Stack Overflow is essentially deprecated for Plone questions. This forum is the best place to ask questions. We do not need a second place (that also frequently closes Plone questions).