Note: You have to be on the Documentation beta in order to contribute.
I have posted a proposal on the Stack Overflow Documentation Beta site for the creation of Magento documentation.
I've been trying this new SO site for a bit and it is quite nice.
If the Magento proposal has passed (currently 1 out of 3 committed), then we can start creating and writing documentation.
Isn't there Magento documentation already at http://docs.magento.com/? Sure there is and it is constantly being improved. The SO Documentation site is all about examples that are lacking in documentation most of the time. Read all about the idea of the Documentation SO here.
Documentation is out of private beta and now in the open. Go here: https://stackoverflow.com/documentation/magento
And support my proposal for Magento docs ;)
20 mins. ago it was created!