Add tooltiop for scroll-to-top button (#829)

Signed-off-by: hossainemruz <hossainemruz@gmail.com>

layouts/_default/single.html

@@ -154,7 +154,9 @@
     </div>
   </div>
   <!--scroll back to top-->
-  <a id="scroll-to-top" class="btn"><i class="fas fa-chevron-circle-up"></i></a>
+  <a id="scroll-to-top" class="btn"  data-toggle="tooltip" data-placement="left" title="Scroll to top">
+    <i class="fas fa-chevron-circle-up"></i>
+  </a>
 </section>
 {{ end }}