Decker doesn't have the ability to access any external resources, like websites. It also can't parse or render HTML. So it's somewhat more limited than Twine in that regard.
In general, all you can do is have a link to a website, using like go["https://itch.io/"], which just prompts the user and then opens the website separately in the browser. You can probably do more stuff if you use the "Danger Zone" features, which can allow for doing stuff like running arbitrary javascript that can pull in whatever, but that's probably not a good option if you're not an experienced coder (and I'd tend to argue it goes against the spirit of doing things 'in Decker').
So I guess the short answer is that what you are trying to do isn't really possible. What are you trying to achieve by embedding a website in Decker?