[DOCS] Add `index-extra-title-page.html` for direct HTML migration (#50189)

docs/reference/index-extra-title-page.html

@@ -0,0 +1,20 @@
+<div class="legalnotice">
+  <p>
+    Welcome to the official documentation for Elasticsearch:
+    the search and analytics engine that powers the Elastic Stack.
+    If you want to learn how to use Elasticsearch to search and analyze your
+    data, you've come to the right place. This guide shows you how to:
+  </p>
+  <div class="itemizedlist">
+    <ul class="itemizedlist" type="disc">
+      <li class="listitem">Install, configure, and administer an Elasticsearch
+      cluster.</li>
+      <li class="listitem">Index your data, optimize your indices, and search
+      with the Elasticsearch query language.
+      </li>
+      <li class="listitem">Discover trends, patterns, and anomalies with
+      aggregations and the machine learning APIs.
+      </li>
+    </ul>
+  </div>
+</div>