A mardown-based wiki under version control and code comments. Everything else likely isn't worth docummenting or just merits direct person-to-person communication.
Deployment details, how to set up test environments, graphical design stuff (e.g.: how to use logo and its palette hex codes, reasoning for some UX decisions, etc), ideas to discuss for major versions... Pretty much everything that needs to be written down and isn't public facing or should remain private to certain positions.