[FLINK-1282] [docs] Make Flink logo link back to frontpage

This closes #237.

docs/_includes/navbar.html

@@ -2,13 +2,13 @@
   <div class="container">
     <div class="row">
       <div class="col-md-1 af-mobile-nav-bar">
-	<a href="index.html" title="Home">
+	<a href="{{ site.baseurl }}/index.html" title="Home">
 	  <img class="hidden-xs hidden-sm img-responsive"
 	       src="{{ site.baseurl }}/img/logo.png" alt="Apache Flink Logo">
 	</a>	
 	<div class="row visible-xs">
 	  <div class="col-xs-3">
-	    <a href="index.html" title="Home">  
+	    <a href="{{ site.baseurl }}/index.html" title="Home">  
 	      <img class="hidden-x hidden-sm img-responsive"
 		   src="{{ site.baseurl }}/img/logo.png" alt="Apache Flink Logo">
 	    </a>