[SPARK-11443] Reserve space lines

The trim_codeblock(lines) function in include_example.rb removes some blank lines in the code.

Author: Xusen Yin <yinxusen@gmail.com>

Closes #9400 from yinxusen/SPARK-11443.

docs/_plugins/include_example.rb

@@ -50,7 +50,7 @@ module Jekyll
         .map { |l| l[/\A */].size }
         .min
 
-      lines.map { |l| l[min_start_spaces .. -1] }
+      lines.map { |l| l.strip.size == 0 ? l : l[min_start_spaces .. -1] }
     end
 
     # Select lines according to labels in code. Currently we use "$example on$" and "$example off$"