add donation button to the sidebar (#464)

Author: namiroues

_includes/sidebar.html

@@ -55,6 +55,11 @@ <h1>
         {% endif %}
       {% endfor %}
     </nav>
+
+    <!-- Donation Button -->
+    <div class="sidebar-donate" style="margin-top: 20px;">
+      {{ site.numfocus.donate_button }}
+    </div>
 
     <p>&copy; {{ site.time | date: '%Y' }}. All rights reserved.</p>
   </div>