Update indexes for better navigation (#3779)

* Update indexes for better navigation

* Fix link

* Fix link

python/packages/autogen-core/docs/src/user-guide/core-user-guide/cookbook/index.md

@@ -1,6 +1,11 @@
 # Cookbook
 
+This section contains a collection of recipes that demonstrate how to use the Core API features.
+
+## List of recipes
+
 ```{toctree}
+:maxdepth: 1
 
 azure-openai-with-aad-auth
 termination-with-intervention

python/packages/autogen-core/docs/src/user-guide/core-user-guide/core-concepts/index.md

@@ -1,5 +1,8 @@
 # Core Concepts
 
+The following sections describe the main concepts of the Core API
+and the system architecture.
+
 ```{toctree}
 :maxdepth: 1
 
@@ -10,4 +13,3 @@ application-stack
 agent-identity-and-lifecycle
 topic-and-subscription
 ```
-

python/packages/autogen-core/docs/src/user-guide/core-user-guide/framework/index.md

@@ -1,16 +1,30 @@
-# Framework
+# Framework Guide
 
+The following sections guides you through the usage of the Core API.
+At minimum, read [Agent and Agent Runtime](agent-and-agent-runtime.ipynb)
+and [Message and Communication](message-and-communication.ipynb) to get the
+basic understanding.
 
+```{note}
+The Core API is designed to be unopinionated and flexible. So at times, you
+may find it challenging. Continue if you are building
+an interactive, scalable and distributed multi-agent system and want full control
+of all workflows.
+If you just want to get something running
+quickly, you may take a look at the [AgentChat API](../../agentchat-user-guide/index.md).
+```
+
+## List of content
 
 ```{toctree}
 :maxdepth: 1
 
 agent-and-agent-runtime
-command-line-code-executors
-distributed-agent-runtime
-logging
 message-and-communication
 model-clients
-telemetry
 tools
-```
+logging
+telemetry
+command-line-code-executors
+distributed-agent-runtime
+```