docs: add maintenance mode notice and recommend Railpack#1378

coffee-cup merged 1 commit intorailwayapp:mainrailwayapp/nixpacks:mainfrom

iloveitaly:claude/update-maintenance-mode-message-011CV4TKAqZ2LeqzcEy5LvHoiloveitaly/nixpacks:claude/update-maintenance-mode-message-011CV4TKAqZ2LeqzcEy5LvHoCopy head branch name to clipboard