Add doc structure to prepare for Public Hub launch

Per #8, and also a first shot at addressing a long-standing lack of doc structure within the documentation Hub. :-P

I mashed together a few things here in my haste; happy to split them up if anyone really insists on it.

@meiqimichelle @leahbannon @avriette: Would appreciate comments in initial structure/clarity. Just shooting for good enough to launch; we can iterate and improve over time.

@afeld: FYI, as always.

@OriHoffer: Please look at the new front page (including the "Updates" section, which I may try to flesh out a little more), and the "About", "Docs", and "Private Docs" sections.

@NoahKunin @gbinal: Anything you'd like to migrate into the "Docs" or "Private Docs" sections in the very near future appreciated. Otherwise, don't sweat it. Let me know if the instructions at the top of those pages aren't clear enough to get started.