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Make Home page slide down

Hi, I want to make an about page that appears behind the home page.
When you click on the about button, the home page starts to slide down 900px on Y axis.
So we see the about page appear behind.

The problem is that the page is not really separate and difficult to design in this way.
Also it doesn’t work on mobile…

I have the feeling that this is not the right way to build this effect.
Anyone has any idea on how to build this in a nice and easy to design way?
Link to the website