I created an account on Stack Exchange documentation beta website a few months ago. It used to appear in my list of Stack Exchange accounts, but now is gone. Why? It seems to affect other (all?) users as well.
Private beta is over. We're moving Documentation back to Stack Overflow. Public beta should begin shortly as well.
As of right now, everyone who had access to the private beta site should also be able to access Documentation on Stack Overflow as well.
This was announced on the beta site, though unfortunately only about a day before the migration began: