Fix button spacing and layout on small screens

Summary:
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Fix buttons issue in small screens

Before:

![image](https://user-images.githubusercontent.com/3904240/29141530-3b5697a0-7d57-11e7-964a-049fb3568bf2.png)

After:

![image](https://user-images.githubusercontent.com/3904240/29141557-67c2780e-7d57-11e7-8e25-c3a8a3f39bc6.png)
Closes https://github.com/facebook/react-native/pull/15437

Differential Revision: D5666978

Pulled By: TheSavior

fbshipit-source-id: 8f2521f4aa5b788f1e6d2743beadb4ac7c2b37cf

website/src/react-native/css/react-native.css

@@ -1013,7 +1013,6 @@ h2 {
   text-rendering: optimizelegibility; }
 
 .docs-prevnext {
-  min-width: 320px;
   max-width: 640px;
   margin: 40px auto;
   padding-bottom: 20px; }
@@ -2048,6 +2047,15 @@ article li {
 .btn:hover {
   text-decoration: none !important; }
 
+@media screen and (max-width: 373px) {
+  .hero .buttons-unit .button {
+    margin-bottom: 4px; }
+
+  .docs-prevnext .btn {
+    margin-bottom: 4px;
+    display: block;
+    float: none; } }
+
 .video-container {
   border-radius: 4px;
   background-clip: padding-box;