Adding block announce for GitHub site deprecation (#597)

* Adding `block announce` for GitHub site deprecation

Author: rodrigobersa

overrides/main.html

@@ -1,2 +1,5 @@
 {% extends "base.html" %}
 
+{% block announce %}
+<h2 style="color:#F2F2F2;text-align: center;"><b> Hi everyone! EKS Best Practices Guide has moved to <a style="color:#ff9900;text-decoration: underline" href="https://docs.aws.amazon.com/eks/latest/best-practices/introduction.html">AWS Documentation</a></u>. This GitHub Pages site is deprecated. </b></h2>
+{% endblock %}