New Game: Gravity Chess

Both players move in the same direction, from top to bottom, trying to stack more pieces on the bottom.

* [x] Write up the rules.
* [x] Add example diagrams.
* [ ] Try moving pieces in order from bottom to top.
* [ ] Try allowing pawns to always move up to two spaces down.
* [ ] Can pawns come unstacked, when an unstacked piece moves into their attack square?
* [ ] Can other pieces come unstacked when they can make a horizontal capture?
* [ ] End when one player has no unstacked pieces at the start of their turn?