Add scheduled maintenance announcement for eksctl.io (#8498)

Author: sapphirew

userdocs/theme/main.html

@@ -2,6 +2,10 @@
 
 {% block announce %}
   <!-- Add announcement here, including arbitrary HTML -->
+  <div style="text-align: center; background-color: #ff6b6b; color: white; padding: 8px; margin-bottom: 4px;">
+    <strong>⚠️ Scheduled Maintenance:</strong> This site will be unavailable on <strong>September 4th, 2025 at 10 PM PDT</strong> for 15 minutes to 1 hour. 
+    Use <a href="https://docs.aws.amazon.com/eks/latest/eksctl/what-is-eksctl.html" style="color: white; text-decoration: underline;">AWS eksctl docs</a> during downtime.
+  </div>
   <p style="text-align: center;">
     The <code>eksctl</code> documentation will be moving from eksctl.io to the 
     <a class="announce" href="https://docs.aws.amazon.com/eks/latest/eksctl/what-is-eksctl.html">official AWS eksctl documentation site</a>.