[DOCS] Updates docs landing page (elastic#100749)

* [DOCS] Updates docs landing page

* [DOCS] Cleans up docs landing page

docs/index-extra-title-page.html

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+<div class="legalnotice">
+  <p>From creating beautiful visualizations to managing the Elastic Stack, learn how Kibana helps you get the most of your data.</p>
+  <table style="width:100%">
+  <tr>
+    <th><h3>New to Kibana?</h3></th>
+    <th><h3>Popular topics</h3></th>
+  </tr>
+  <tr>
+    <td>
+      <div class="itemizedlist">
+        <ul class="itemizedlist" type="disc">
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/introduction.html">What is Kibana?</a></li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/get-started.html">Quick start</a>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/dashboard.html">Create a dashboard</a></li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/connect-to-elasticsearch.html">Add data</a></li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/discover.html">Search your data</a>
+          </li>
+        </ul>
+      </div>
+    </td>
+    <td>
+      <div class="itemizedlist">
+        <ul class="itemizedlist" type="disc">
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/install.html">Install Kibana</a></li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/settings.html">Configure settings</a></li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/kuery-query.html">Kibana Query Language (KQL)</a>
+          </li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/index-patterns.html">Create an index pattern</a>
+          </li>
+          <li class="listitem"><a href="https://www.elastic.co/guide/en/kibana/current/alerting-getting-started.html">Create an alert</a>
+        </ul>
+      </div></td>
+  </tr>
+</table>
+<h3>All topics</h3>
+</div>