The Holy Isle of Leng (Origins of the Green Men)

We're told very little about the Sacred Order of Green Men on the Isle of Faces - like, what are they even? But we know the Isle of Faces is full of weirwoods, and is probably important to the story. Good news! I think I've solved this most obscure of origin mysteries in ASOIAF, and the story begins in the far east, on the Holy Isle of Leng... or, rather, beneath it...