Added Chat app plus explainer video

Author: martygubar

apex/README.md

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+# Sample APEX Apps
+Listed below are sample applications that leverage Autonomous Database.
+
+## ADB Chat
+ADB Chat allows you to query data in Autonomous Database using natural language. It uses the AI profiles that are in the current user's schema. Those profiles support both natural language to SQL as well as RAG:
+* [Create an AI profile](../sql/select-ai-create-profile.sql)
+* [NL2SQL](../sql/select-ai-nl2sql.sql)
+* [Select AI RAG](../sql/select-ai-rag.sql)
+
+![ADB Chat APEX app](./select-ai-chat-across-subject-areas.mp4)
+
+
+<hr>
+Copyright (c) 2025 Oracle and/or its affiliates.<br>
+Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl/