SSEO-3393 - Add metatag for Zendesk search crawler

https://confluentinc.atlassian.net/browse/SSEO-3393

In order to make content from https://docs.ksqldb.io searchable via Zendesk Federated Search in Support Portal (https://support.confluent.io/hc), we need to add a meta tag to this website.

Zendesk's search crawler uses this tag for domain verification each time it runs and more info about the crawler setup can be found in https://support.zendesk.com/hc/en-us/articles/4593564000410

docs/overrides/main.html

@@ -3,4 +3,5 @@
 {% block extrahead %}
   <meta class="swiftype" name="site-id" data-type="integer" content="4" />
   <meta class="swiftype" name="docs-boost" data-type="integer" content="6" />
+  <meta name='zd-site-verification' content='30d1zoyi0fpyh1acggoi58' />
 {% endblock %}