For example, kattio, which I found in USACO guide.
I don’t think so.
You may therefore NOT use pre-written code or “templates” to get a head start on your coding, and you may not consult resources other than those that provide information about basic functionality of your programming language (e.g., syntax, library functions, input / output, etc.).
You could argue that the input-output module satisfies this condition, so you’re allowed to consult it. Though it seems that you’re not allowed to copy-paste from it, as that would count as “pre-written code” …
Does that mean you can visit the official documentation for your specific language?
Yes.
That’s good. For AIO (Australian Informatics Olympiad), we’re allowed templates which are already given from the contest website but we cannot get templates or pre-wrtten code outside the contest website.