The key thing for any links in any SE content is that the question or answer must still be sensible and useful even if the link goes dead. So if you are asking about the thing in the link, then that’s a problem.
If you are, instead, asking about something that builds off of another resource, which you link to, that doesn’t have to be fully reproduced—answerers are supposed to have some expertise with the material in question anyway, so even if the link goes dead the people doing the answering should be familiar with it. This works well if it’s a bigger production that people are aware of, but even with something smaller it might be a reasonable trade-off for the sake of not infringing on anyone’s copyright, keeping the question to a practicable size (both for the author and the reader), and focusing on your actual content to review. Some basics like “the classes in this product have whatsits that work kind of like spells except...” would be a good compromise.
Also, bear in mind, that there is another concern beyond simple rule-following: you want an answer. The easier you make the question to answer, the more likely you are to get one. Clicking on a link to somewhere else can be a turn-off for a prospective answerer; you might have better luck if you reproduce more of the material in the question itself. Likewise, copying and pasting is often part of reviewing the material, so putting it in actual, selectable text rather than, say, a screenshot embedded in the question, improves the likelihood that you’ll get answers. Homebrew-review questions also seem to attract downvotes pretty easily, while attracting fewer positive readers (some people just dislike them as a rule, I think, while other people are fine with them but also not particularly interested in trying to actually perform the review and so don’t click on the question): the more you seem to be engaging with the SE format and community, the more you seem to be putting your money where your mouth is, so to speak, by putting in the effort, the less that will happen. That’s important, too, because downvoted questions often get less attention, and thus are less likely to see an answer.
