Add Join PrestoDB on Slack link on doc pages

Adding a link in the document pages to encourage users to
join the PrestoDB Slack when seeking help.

Signed-off-by: Yihong Wang <yh.wang@ibm.com>

presto-docs/src/main/sphinx/_templates/partials/toc.html

@@ -40,6 +40,14 @@
                 Create project issue
             </a>
         </li>
+        <li class="md-nav__item">
+            <a href="https://communityinviter.com/apps/prestodb/prestodb" class="">
+                <span class="md-source__icon  md-icon">
+                    {% include ".icons/fontawesome/brands/slack.svg" %}
+                </span>
+                Join PrestoDB on Slack
+            </a>
+        </li>
     </ul>
     <hr>
     {% set toc = page.toc %}