feat(docs): Add Slack link to footer

docs/layouts/partials/footer.html

@@ -8,6 +8,7 @@
                     <li><a href="https://github.com/vendure-ecommerce/vendure">GitHub</a></li>
                     <li><a href="https://www.npmjs.com/~vendure">npm</a></li>
                     <li><a href="https://twitter.com/vendure_io">Twitter</a></li>
+                    <li><a href="https://join.slack.com/t/vendure-ecommerce/shared_invite/enQtNzA1NTcyMDY3NTg0LTMzZGQzNDczOWJiMTU2YjAyNWJlMzdmZGE3ZDY5Y2RjMGYxZWNlYTI4NmU4Y2Q1MDNlYzE4MzQ5ODcyYTdmMGU">Join us on Slack</a></li>
                     <li><a href="mailto:contact@vendure.io" class="email-link"><img src="{{ "svg/clr-icon-email-light.svg" | relURL }}"> contact@vendure.io</a></li>
                 </ul>
             </div>