Link to HugeGraph-specific security page from footer (#363)

Make the project-specific security page more prominent by
linking to it from the website footer.

(also changed some links to https)

themes/docsy/layouts/partials/footer.html

@@ -12,12 +12,12 @@
  </div>
  </a>
  <ul class="footer-link">
- <li><a class="white" href="http:https://www.apache.org">Foundation</a></li>
- <li><a class="white" href="http:https://www.apache.org/licenses/">License</a></li>
- <li><a class="white" href="https://www.apache.org/security/">Security</a></li>
- <li><a class="white" href="http:https://www.apache.org/events/current-event">Events</a></li>
- <li><a class="white" href="http:https://www.apache.org/foundation/sponsorship.html">Sponsorship</a></li>
- <li><a class="white" href="http:https://www.apache.org/foundation/thanks.html">Thanks</a></li>
+ <li><a class="white" href="https:https://www.apache.org">Foundation</a></li>
+ <li><a class="white" href="https:https://www.apache.org/licenses/">License</a></li>
+ <li><a class="white" href="https://hugegraph.apache.org/docs/guides/security">Security</a></li>
+ <li><a class="white" href="https:https://www.apache.org/events/current-event">Events</a></li>
+ <li><a class="white" href="https:https://www.apache.org/foundation/sponsorship.html">Sponsorship</a></li>
+ <li><a class="white" href="https:https://www.apache.org/foundation/thanks.html">Thanks</a></li>
  <li><a class="white" href="https://privacy.apache.org/policies/privacy-policy-public.html" target="_blank">Privacy</a></li>
  </ul>
  </div>