[SPARK-11297] Add new code tags

mengxr https://issues.apache.org/jira/browse/SPARK-11297

Add new code tags to hold the same look and feel with previous documents.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #9265 from yinxusen/SPARK-11297.

docs/css/main.css

@@ -74,6 +74,10 @@ code {
   color: #444444;
 }
 
+div .highlight pre {
+  font-size: 12px;
+}
+
 a code {
   color: #0088cc;
 }