Make <Background /> component responsive (#4767)

- Adjusted the height of the background image to fit every possible
screen
- Added the proper rotation degree to the background
- Refactored gradient colour stop into CONST to make sure each
background image starts at the same level

**To be noted :** 
I had to use Javascript and useEffect to make the div take the entire
height of the page (turn it into a "use client" component). This was
necessary because absolute positioning by default makes the element's
sizing relative to its nearest positioned ancestor, not the entire
document.

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Co-authored-by: Ady Beraud <a.beraud96@gmail.com>

packages/twenty-website/src/app/_components/contributors/AvatarGrid.tsx

@@ -26,6 +26,7 @@ const AvatarItem = styled.div`
   border-radius: 16px;
   overflow: hidden;
   transition: 200ms;
+  background-color: white;
 
   &:hover {
     -webkit-box-shadow: -6px 6px 0px 1px rgba(0, 0, 0, 1);