Is possible to set a itch.io page for a game design/draft? Maybe there are guidelines about that?

if there's already any, can anyone give me the link to another game concept design/draft here? :)

Maybe if you file it under "everything else", but I'm not aware of any examples. Just classify your uploads suitably, so the site and app don't mistake them for executable files.

For what it's worth, Open Game Art accepts (design) documents as a specific type of submission, but only if they're open source. And there aren't many examples there either.

I think using the "Other" category makes the most sense for this. When uploading files you can specify whether they're a document or something other than a game executable.

I don't know any other game design documents on itch.io off the top of my head, but I'm sure I've seen stuff like that before. We're pretty okay with people uploading different kinds of things as long as they aren't classified incorrectly.