My player guides tend to run three pages
Page 1: setting stuff
A four sentence max “elevator pitch” to grab attention, sprinkle the themes of the starting arc(s)/setting - basically your move trailer.
Then a couple paragraphs at history, the first paragraph giving some older history that describes some major points that define the setting ( war, great schisms, etc) and the second paragraph starts bleeding to more recent events that might tie indirectly to the first story arc.
a sentence or four about religion if needed ( my players always ask) even if that is to say ‘ most major religions are there’ keeping it a bit open ended for players to pick a deity they want and refine the list through play emergence.
Page2: character generation specific rules , such as house rules or calling attention to the rules for new players, ‘starting ability scores: generate as normal; classes: regular available though if picking a cleric can only choose from x/y/z domains. Races: note only using the standard human not the optional one for this campaign. Starting equipment as normal plus one extra healing potion. Available books for this campaign: Tasha’s plus core phb. And so on,
Page 3
Any general game optional rules that you are going to use this campaign, the stuff not specific to character generation but every day play. Such as this campaign will use (this) for critical hits and this is a critical failure table being used for this campaign
That’s just my general lay out, add or subtract as needed. Do you want an additional page for a region map? Do you want an additional page of history? Are you not using any real optional rules and just sticking with core standard . Maybe a page just on major factions/churches/guilds
Etc etc. regardless a page of history ( fluff) and a page on rules related stuff ( crunch) at minimum even if each page is half blank, keep it separate. And at max 5 pages grand total is my personal max, if I go over that I know I need to trim down and find ways to translate some of that in to in game ‘show’ rather than document ‘tell’
But for your own benefit use spacing, headers, bullet lists, and so on. And no more than two pages of history text is about my players’ limit and 5 page total max , but your group can/will vary.