Prepositions are one of the most inconsistent areas for translation. All it takes for me to remember that is to remind myself that while I was "in college" here in the US, my Canadian friends were "at University". We quite literally mean the same thing.

StackEdo is technically correct in suggesting je, although it isn't the best choice. Je is the preposition to use when there isn't a better one. It's not the best choice here, but it would be understood. As a fallback when you aren't sure, you can use it.

However, I agree that en is the best choice. Ĉe is good, but I have a reason for choosing en over it. When we talk about websites, it's common to refer to their "content". Content is something that is contained in something. That's the primary use of "in" in English and "en" in Esperanto. The content of a website is "in" the website in much the same way that articles are in a magazine. The best argument for any choice of a translation is always one based on the concept being translated, not just the word.