Optimize hero images, feather edges

Author: alancwoo

docs/.vuepress/theme/components/Hero.vue

@@ -1,6 +1,7 @@
 <template>
   <div class="hero relative content-block">
-    <img alt="Ethereum.org Hero Image" height="auto" src="../images/hero.png" />
+      <img alt="Ethereum.org Hero Image" class="hide-dark" width="946" height="auto" src="../images/hero.gif" />
+      <img alt="Ethereum.org Hero Image - Dark" class="show-dark" width="946" height="auto" src="../images/hero-dark.gif" />
 
     <div class="hero-block beginners sm-hide">
       <router-link class="block header" to="/beginners/">→ Beginners</router-link>