Make cards on homepage stack on mobile.

Author: BigBlueHat

index.liquid

@@ -70,7 +70,7 @@ masthead:
   listed below is fully conforming to the official JSON-LD specifications.
 </p>
 
-<div class="ui four cards">
+<div class="ui four stackable cards">
   {% include 'implementation-card' language: 'JavaScript' %}
   {% include 'implementation-card' language: 'Python' %}
   {% include 'implementation-card' language: 'Ruby' %}
@@ -141,7 +141,7 @@ masthead:
   </p>
 </div>
 
-<div class="ui four cards">
+<div class="ui four stackable cards">
   <div class="card">
     <div class="content">
       <div class="header"><h3>Code</h3></div>