docs: Add deprecation message for v3

Author: d3xter666

overrides/main.html

@@ -0,0 +1,12 @@
+{#-
+  This file was initially copied from https://squidfunk.github.io/mkdocs-material/customization/#overriding-blocks
+  It displays a message for the non-default branch
+-#}
+{% extends "base.html" %}
+
+{% block announce %}
+  ⚠️ UI5 CLI v3 has been deprecated! We highly recommend migrating to
+  <a href="{{ base_url ~ '/../stable/' }}">
+  <strong>the latest version.</strong>
+  </a>
+{% endblock %}