Remove everything under documention subdir, just provide a pointer to the wiki/yard

See #4000 -- some of the docs haven't been touched in forever, and the wiki is a far better source of info these days -- not to mention the secret *MetaKitty* project due to go live at the end of the month.

/cc @tdoan-r7 and @wchen-r7 -- I'd like to know what you think of just nuking all this. Is anything of value lost? If so, can we host it somewhere?