Ah, I see. Well the short answer is however you want. Just as you can deliver a game in whatever package you want (Windows .exe, HTML5 package, Android .apk, Mac .app, etc.) for whatever platforms you want to prioritize, even though HTML or Windows are recommended to hit the most players, you can naturally publish concept materials in whatever format is best for your game.
This is genuinely just whatever files you'd make for yourself and your team to brainstorm your game concept to yourselves; all you're doing is exposing your process.
If you're an art-driven team, maybe you just make several pieces of concept art and upload those to an itch page as screenshots, or if you're used to presenting games in a pitch document-like format, maybe you upload a PowerPoint file that goes through the overall plan for the game. If you're a hands-on person who likes writing pen-and-paper notes or making a dream board of sticky notes to brainstorm, take a picture of that!
However, on average I'd expect most people to write their concepts in a Google Document (especially with a team), perhaps with some reference material images or links embedded, or concept art embedded within the document. Then you could simply link to that Google Document, and/or upload it as a Word or PDF.
Anything is fine. As long as it's a file(s) other people can open and examine, it can be in whatever format. However, if you want it to be easily seen by as many people as possible to share your process, I'd recommend Google Docs and/or Google Slides.