Link to Druid-specific security page

Add a 'Security' link to the homepage pointing to the
Druid-specific security overview page, which contains
relevant information about the Druid security trust model,
and also contains the information on how to report security
issues.

_includes/page_header.html

@@ -20,7 +20,6 @@
             <a href="https://www.apache.org/events/current-event" target="_blank">Events</a>
             <a href="https://www.apache.org/licenses/" target="_blank">License</a>
             <a href="https://www.apache.org/foundation/thanks.html" target="_blank">Thanks</a>
-            <a href="https://www.apache.org/security/" target="_blank">Security</a>
             <a href="https://www.apache.org/foundation/sponsorship.html" target="_blank">Sponsorship</a>
           </div>
         </li>

index.html

@@ -148,6 +148,13 @@ <h5>Quickstart</h5>
             <a href="/docs/latest/tutorials/quickstart">Get started with Druid</a> in minutes. Load your own data and query it.
           </p>
         </div>
+        <div class="feature">
+          <span class="fa fa-shield fa"></span>
+          <h5>Security</h5>
+          <p>
+            Learn about <a href="https://druid.apache.org/docs/latest/operations/security-overview.html">security features, configuration instructions, and best practices</a> to secure Druid.
+          </p>
+        </div>
         <div class="feature">
           <span class="fa fa-question-circle fa"></span>
           <h5>Get Help</h5>