New players decide whether a game is hard before they finish page one. That judgment rarely rests on the rules themselves. It rests on how the first page feels: an invitation to sit down, or a demand to study first.
So page one has three jobs. First, say how to win, in one sentence, at the top. Once players know the goal, every later rule has a reason. Second, say what a turn is, in order, in five steps or fewer. Third, say where the rest of the rules live: which page covers edge cases, which covers scoring, which lists the components.
Plenty of things do not belong on page one. The setting can go on the back cover. Designer thanks can go at the end. Rare situations can go in an appendix. They all matter; they just do not belong to the first turn.
When we edit a rulebook, we take page one out on its own and hand it to someone who has never seen the game. If they can then tell us how to win and what a turn is, page one is done.