Update REAMDE to reference Microsoft Learn

[zhiyuanliang-ms]

Why this PR?

The content of README has been migrated to the Microsoft Learn: https://learn.microsoft.com/en-us/azure/azure-app-configuration/feature-management-dotnet-reference?pivots=stable-version

The README in this repo should reference Microsoft Learn.

[rossgrambo]

I think we should offer a slightly more explicit link to the docs like in the ASP.NET readme

They have the same descriptive paragraph with a link at the end- but then they have a quick bullet list with some main doc urls. Even if it's just something like:

• .NET Feature Management Overview
• API Reference

The customer I have in mind is someone who's familiar with FM but wants to get to a specific feature- like variants. The paragraph isn't relevant to them and the current link seems like it's a part of the intro.

[zhiyuanliang-ms]

@rossgrambo

they have a quick bullet list with some main doc urls. Even if it's just something like:

Do you mean the Get started section in the ASP.NET readme

How about this: we also add a Get started section in our readme. We will list the following topics: Concepts: https://learn.microsoft.com/en-us/azure/azure-app-configuration/concept-feature-management (This page needs to be updated. It should reference the new .NET feature management page) feature flag configuration: https://learn.microsoft.com/en-us/azure/azure-app-configuration/feature-management-dotnet-reference?pivots=stable-version#feature-flag-configuration How to use feature management: https://learn.microsoft.com/en-us/azure/azure-app-configuration/feature-management-dotnet-reference?pivots=stable-version#consumption

[zhiyuanliang-ms]

I have a question in my mind: who will read this README? Newcomers who just find this feature management library or pro users who have been using the library and want to have a deeper understand of this repo? For the newcomers, we can write a simple example in the readme and give some links to the Microsoft Learn (we have plenty of quickstarts there). For the pro users, I think a detailed index will be helpful. How about keep the original index section here:

## Index
* [Feature Flags](#feature-flags)
    * [Feature Filters](#feature-filters)
    * [Feature Flag Declaration](#feature-flag-declaration)
* [Consumption](#consumption)
* [ASP.NET Core Integration](#ASPNET-core-integration)
* [Implement a Feature Filter](#implementing-a-feature-filter)
* [Providing a Context For Feature Evaluation](#providing-a-context-for-feature-evaluation)
* [Built-in Feature Filters](#built-in-feature-filters)
* [Targeting](#targeting)
  * [Targeting Exclusion](#targeting-exclusion)
* [Caching](#caching)
* [Custom Feature Providers](#custom-feature-providers)

[zhenlan]

How about keep the original index section here

I would not duplicate that content here. It adds extra burden for maintenance.

If we like, we can have something like this:

Follow the [quickstart]() guide to learn how to integrate feature flags from Azure App Configuration into your .NET applications. Check out the [feature reference]() document for a full feature rundown. See the [API reference]() for more information.

or shorter version:

Follow the [quickstart]() for .NET feature flag integration. View the [feature reference]() and [API reference]() for further details.

(The links are placeholders only)