Hey @Sbi - in these cases we normally designate the top group of the board and a single item for the storage of project/board level files.
Another way to look at it could be to create an Overview Board, which is basically a one-liner per project that has all the high-level details of the project. This is a good location to store related files. You could then also mirror back tasks status to the overview board for a have an easy sense of all project health.
Hope this helps,
Mark