docs: add index page for learn section (#18777)

(cherry picked from commit 1a49605)

# Conflicts:
#	docs/docs/learn/learn.md

docs/docs/learn/advanced/_category_.json

@@ -1,5 +1,5 @@
 {
     "label": "Advanced",
-    "position": 2,
+    "position": 3,
     "link": null
 }

docs/docs/learn/beginner/_category_.json

@@ -1,5 +1,5 @@
 {
     "label": "Beginner",
-    "position": 1,
+    "position": 2,
     "link": null
 }

docs/docs/learn/intro/_category_.json

@@ -1,5 +1,5 @@
 {
     "label": "Introduction",
-    "position": 0,
+    "position": 1,
     "link": null
 }

docs/docs/learn/learn.md

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+---
+sidebar_position: 0
+---
+# Learn
+
+*   [Introduction](intro/00-overview.md) -  Dive into the fundamentals of Cosmos SDK with an insightful introduction,
+laying the groundwork for understanding blockchain development. In this section we provide a High-Level Overview of the SDK, then dive deeper into Core concepts such as Application-Specific Blockchains, Blockchain Architecture, and finally we begin to explore what are the main components of the SDK.
+*   [Beginner](beginner/00-app-anatomy.md) - Start your journey with beginner-friendly resources in the Cosmos SDK's "Learn"
+section, providing a gentle entry point for newcomers to blockchain development. Here we focus on a little more detail, covering the Anatomy of a Cosmos SDK Application, Transaction Lifecycles, Accounts and lastly, Gas and Fees.
+*   [Advanced](advanced/00-baseapp.md) - Level up your Cosmos SDK expertise with advanced topics, tailored for experienced
+developers diving into intricate blockchain application development. We cover the Cosmos SDK on a lower level as we dive into the core of the SDK with BaseApp, Transactions, Context, Node Client (Daemon), Store, Encoding, gRPC, REST, and CometBFT Endpoints, CLI, Events, Telementry, Object-Capability Model, RunTx recovery middleware, Cosmos Blockchain Simulator, Protobuf Documentation, In-Place Store Migrations, Configuration and AutoCLI.