Sorry, have been away for a full month :D
The Documentation site is a separate site within the StackExchange network and is not only a sub-site for StackOverflow (although it is located there). It can facilitate documentation on all kinds of topics (as you can see) from big names like PHP/.NET/SQL/Java to "never-heard-of-it" stuff like Jekyll/lodash/zeromq. Documentation sections (tags) are created by the community and thus I've proposed a Magento tag/documentation.
So what is the Documentation's place in all that's Magento? There's Magento SE for specific Q&A and there's the Magento official Docs for basic walk-through style reads. Documentation falls in between, with an overlap here and there. Documentation is all about giving many examples on general cases. So it is not as specific as Q&A, and where the official docs may lack examples on every use case, the SE Documentation site can provide that.
Also it allows for examples of full modules, something that will be flagged as too broad here on the Q&A.
In addition to that, you are encouraged to link to the documentation sections. This will the writer/contributer of a documentation topic a reputation bonus for being referenced (cited): https://stackoverflow.com/help/whats-reputation (look for "cite")
There's a lot of meta discussions on the new Documentation site, so feel free to browse https://meta.stackexchange.com/ for it