docs(devcenter): improve Azure.ResourceManager.DevCenter README with key concepts and examples

## Summary

The `Azure.ResourceManager.DevCenter` package was recently migrated from AutoRest/Swagger to the TypeSpec-based management plane generator (MPG). Its README was left as a generic boilerplate with no service-specific documentation, examples, or key concepts. This PR closes that documentation gap.

## Changes

### `README.md`
- **Rewrote the introduction** to accurately describe Azure Dev Center and its role in managing developer environments.
- **Added a Key Concepts table** covering the resource hierarchy: DevCenter → Project → DevBoxDefinition / Pool / NetworkConnection / Catalog / EnvironmentType.
- **Added six code examples** that demonstrate the most common management operations:
1. Create a Dev Center (including resource group setup)
2. Get an existing Dev Center
3. List all Dev Centers in a resource group
4. Create a Project linked to a Dev Center
5. Create a Dev Box Definition (image + SKU)
6. Create a Pool (links a definition and network connection to a project)
7. Create a Network Connection (Azure AD join)
8. Delete a Dev Center
- All code blocks use `C# Snippet:` markers so they are backed by compilable test files and can be validated by `eng/scripts/Update-Snippets.ps1`.
- Added a link to the [Azure Dev Box product docs]((learn.microsoft.com/redacted)

### `CHANGELOG.md`
- Documented the MPG migration and README improvements in the `1.1.0-beta.1 (Unreleased)` section.

### `tests/Samples/Sample1_DevCenterOperations.cs` (new)
Snippet-backed sample covering DevCenter CRUD operations.

### `tests/Samples/Sample2_ProjectOperations.cs` (new)
Snippet-backed sample covering Project, DevBoxDefinition, Pool, and NetworkConnection creation.

## Validation

The sample files compile against the existing `Azure.ResourceManager.DevCenter` package types. Snippet names are unique and every snippet referenced in the README has a corresponding `#region Snippet:` block in the sample files.

> Generated by [Update Docs](https://github.com/Azure/azure-sdk-for-net/actions/runs/23938838103/agentic_workflow) · [◷](https://github.com/search?q=repo%3AAzure%2Fazure-sdk-for-net+%22gh-aw-workflow-id%3A+update-samples-and-docs%22&type=pullrequests)

---

> [!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/devcenter-mgmt-readme-update-a98df0910f4a485c`.
>
> **[Click here to create the pull request](https://github.com/Azure/azure-sdk-for-net/compare/main...docs/devcenter-mgmt-readme-update-a98df0910f4a485c?expand=1&title=docs(devcenter)%3A%20improve%20Azure.ResourceManager.DevCenter%20README%20with%20key%20concepts%20and%20examples)**

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Show patch preview (500 of 547 lines)

```diff
From 81828a6f3cd518673773ef3a3f57d4d3bb1a8560 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
Date: Fri, 3 Apr 2026 08:01:34 +0000
Subject: [PATCH] docs(devcenter): improve Azure.ResourceManager.DevCenter
README with key concepts and examples

- Rewrote README with DevCenter-specific introduction, resource hierarchy
table (DevCenter, Project, DevBoxDefinition, Pool, NetworkConnection,
Catalog, EnvironmentType), and product documentation links.
- Added six runnable code examples backed by snippet markers:
CreateDevCenter, GetDevCenter, ListDevCenters, CreateProject,
CreateDevBoxDefinition, CreatePool, CreateNetworkConnection, DeleteDevCenter.
- Created tests/Samples/Sample1_DevCenterOperations.cs and
tests/Samples/Sample2_ProjectOperations.cs containing the #region Snippet
blocks that back the README code blocks.
- Updated CHANGELOG 1.1.0-beta.1 (Unreleased) to document the MPG migration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
.../CHANGELOG.md | 3 +
.../Azure.ResourceManager.DevCenter/README.md | 229 +++++++++++++++---
.../Samples/Sample1_DevCenterOperations.cs | 91 +++++++
.../Samples/Sample2_ProjectOperations.cs | 121 +++++++++
4 files changed, 409 insertions(+), 35 deletions(-)
create mode 100644 sdk/devcenter/Azure.ResourceManager.DevCenter/tests/Samples/Sample1_DevCenterOperations.cs
create mode 100644 sdk/devcenter/Azure.ResourceManager.DevCenter/tests/Samples/Sample2_ProjectOperations.cs

diff --git a/sdk/devcenter/Azure.ResourceManager.DevCenter/CHANGELOG.md b/sdk/devcenter/Azure.ResourceManager.DevCenter/CHANGELOG.md
index e73a6bea3..5aedfc396 100644
--- a/sdk/devcenter/Azure.ResourceManager.DevCenter/CHANGELOG.md
+++ b/sdk/devcenter/Azure.ResourceManager.DevCenter/CHANGELOG.md
@@ -10,6 +10,9 @@

### Other Changes

+- Migrated SDK code generation from AutoRest to the new TypeSpec-based management plane generato
... (truncated)
```