Adds link to Robinhood

docs/.templates/sidebarintro.html

@@ -14,5 +14,7 @@
 
 <div id="sponsors">
     <b>Sponsored by:</b>
-    <img src="https://s3-us-west-1.amazonaws.com/celerydocs/images/robinhood_logotype_green_left.png" />
+    <a href="https://robinhood.com">
+        <img src="https://s3-us-west-1.amazonaws.com/celerydocs/images/robinhood_logotype_green_left.png" />
+    </a>
 </div>

docs/.templates/sidebarlogo.html

@@ -14,5 +14,7 @@
 
 <div id="sponsors">
     <b>Sponsored by:</b>
-    <img src="https://s3-us-west-1.amazonaws.com/celerydocs/images/robinhood_logotype_green_left.png" />
+    <a href="https://robinhood.com">
+        <img src="https://s3-us-west-1.amazonaws.com/celerydocs/images/robinhood_logotype_green_left.png" />
+    </a>
 </div>