Hi @LucTango - the basics of getting started with custom pages can be found here:
Would you need some of your own HTML for the content to look clean. This isn’t a use case for custom pages that has come up before, so I’m afraid I don’t have a template for that (making it look like an article). But it should be fairly straightforward.
Also, technically the page would be open to everyone rather than restricted to those with a direct link, but the reason I named this as a potential solution is that in practice the page won’t be easily findable if you don’t promote it with links or navigation elements. I’m not sure if this is a good solution for you - it’s just the only option I could think of to get close to achieving what you described. :)