Do not load hero image on every slide

[nop33]

I removed framer motion animations and replaced them with a simple opacity transition.

@mvaivre feel free to play around with the following values to adjust the transition speed of the different layers:

  &.front {
    transition-duration: 0.1s;
  }

  &.middle {
    transition-duration: 0.3s;
  }

  &.back {
    transition-duration: 0.5s;
  }

I also implemented an initial fade-in of the image, not sure it affects our Lighthouse score, testing now... all good, nothing changed:

Here's a preview: https://www2.alephium.org