Add CC0 license for content

[zkat]

Would be good to edit LICENSE.txt such that the fact that all the text in _pages (our policy docs in particular) are actually CC0-1.0 (aka "public domain-ish"). I thought I'd licensed this before but I probably forgot, even though it's always been intended as CC0.

Might have to do something different-ish ("CC0 but keep this attribution footer") for the CoC or something, since it's a derivative.