custom-theme: update footer

Add Privacy Policy button to footer.
All ASF websites must link to the Privacy Policy.

Author: piotrnarajowski

custom-theme/footer.html

@@ -51,4 +51,9 @@
             ASF Events
         </button>
     </a>
+    <a href="https://www.apache.org/foundation/policies/privacy.html">
+        <button class="button-footer-asf">
+            Privacy Policy
+        </button>
+    </a>
 </footer>