fix: move particles on landing page behind content on page (#133)

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# Description

Move the particles animation (on the landing page) to behind the content
displayed

## Linked Issues

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## Additional context


![image](https://github.com/matter-labs/zksync-docs/assets/1890113/cecd6d10-4df2-4d46-8ed3-11a6599f254a)

pages/index.vue

@@ -28,6 +28,7 @@ defineOgImage({
         <vue-particles
           id="tsparticles"
           url="particles.json"
+          style="z-index: -10"
         />
       </ClientOnly>
       <template #headline>