docs: improve README for Azure.ResourceManager.KubernetesConfiguration.Extensions

## Summary

The `Azure.ResourceManager.KubernetesConfiguration.Extensions` package was introduced in [#57720](https://github.com/Azure/azure-sdk-for-net/pull/57720) (MPG migration) with a minimal, generic README that was essentially a copy of the base `Azure.ResourceManager.KubernetesConfiguration` README.

This PR improves the README to provide:

### Changes

- **Better description**: Explains what the package does (manage Kubernetes Cluster Extensions on AKS and Arc-enabled clusters) and lists common extension types (Flux/GitOps, Container Insights, Azure Policy).
- **Key concepts section**: Explains `KubernetesClusterExtensionResource` and the three required cluster-identification parameters (`clusterRp`, `clusterResourceName`, `clusterName`) that are unique to this API.
- **Five practical code examples** covering all major CRUD operations:
1. Create a cluster extension
2. Get a cluster extension by name
3. List all extensions on a cluster
4. Update (patch) a cluster extension
5. Delete a cluster extension
- **Improved links**: Updated documentation links to include the [Azure Arc Kubernetes Extensions overview]((learn.microsoft.com/redacted)

### Motivation

Documentation gaps are treated like failing tests. The original README provided no guidance on how to use the package, leaving developers without any starting point. The new README follows the progressive-disclosure pattern: setup → authentication → concepts → working code examples.

### Package scope

Changes are limited to the `sdk/kubernetesconfiguration/Azure.ResourceManager.KubernetesConfiguration.Extensions/` directory, matching the management package introduced in #57720.

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---

> [!NOTE]
> This was originally intended as a pull request, but GitHub Actions is not permitted to create or approve pull requests in this repository.
> The changes have been pushed to branch `docs/kubernetes-configuration-extensions-readme-17a0cc35e18b1148`.
>
> **[Click here to create the pull request](https://github.com/Azure/azure-sdk-for-net/compare/main...docs/kubernetes-configuration-extensions-readme-17a0cc35e18b1148?expand=1&title=docs%3A%20improve%20README%20for%20Azure.ResourceManager.KubernetesConfiguration.Extensions)**

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```diff
From 58f48e1bc3ae689843edd321169227711edc664c Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
Date: Fri, 3 Apr 2026 08:27:39 +0000
Subject: [PATCH] docs: improve README for
Azure.ResourceManager.KubernetesConfiguration.Extensions

Add detailed description, key concepts section explaining cluster extension
parameters (clusterRp, clusterResourceName, clusterName), and practical
code examples for all major CRUD operations:
- Create a cluster extension
- Get a cluster extension
- List all extensions on a cluster
- Update (patch) a cluster extension
- Delete a cluster extension

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
.../README.md | 207 ++++++++++++++----
1 file changed, 170 insertions(+), 37 deletions(-)

diff --git a/sdk/kubernetesconfiguration/Azure.ResourceManager.KubernetesConfiguration.Extensions/README.md b/sdk/kubernetesconfiguration/Azure.ResourceManager.KubernetesConfiguration.Extensions/README.md
index 22532b456..1a7b9ec6f 100644
--- a/sdk/kubernetesconfiguration/Azure.ResourceManager.KubernetesConfiguration.Extensions/README.md
+++ b/sdk/kubernetesconfiguration/Azure.ResourceManager.KubernetesConfiguration.Extensions/README.md
@@ -1,20 +1,20 @@
-# Microsoft Azure Kubernetes Configuration Extensions management client library for .NET
+# Azure Resource Manager Kubernetes Configuration Extensions client library for .NET

-This library supports managing Microsoft Azure Kubernetes Configuration Extensions resources.
+The Azure Kubernetes Configuration Extensions management library allows you to manage [Kubernetes Cluster Extensions](https://learn.microsoft.com/azure/azure-arc/kubernetes/conceptual-extensions) on AKS (Azure Kubernetes Service) and Arc-enabled Kubernetes clusters. Extensions allow you to install and manage lifecycle of cluster-level services such as monitoring, Azure Policy, Flux (GitOps), and more.

This library follows the [n
... (truncated)
```