Refactor hero into grid-uneven and large-only classes

This makes it possible to reuse this CSS for the about page layout.

index.html

@@ -3,8 +3,8 @@ title: CoTech
 layout: default
 ---
 
-<section class="hero">
-  <div class="hero__left stack">
+<section class="grid grid-uneven">
+  <div class="stack">
     <h1>We're building a cooperative digital economy</h1>
 
     <p class="large">CoTech is a UK-wide network of creative technology companies
@@ -21,7 +21,7 @@ layout: default
     </div>
   </div>
 
-  <div class="hero__right">
+  <div class="large-only">
     <img src="assets/img/decoration.svg" alt="" />
   </div>
 </section>