Add introduction to what's new in Steeltoe 4

[bart-vmware]

Follow-up for #337 (comment)

[TimHess]

Suggested change

	Steeltoe started as a C# port of Java Spring Boot parts, offering .NET interoperability with enterprises running
	[Spring Boot](https://spring.io/projects/spring-boot) applications on [Cloud Foundry](https://www.cloudfoundry.org/) and [Tanzu Platform](https://www.vmware.com/products/app-platform/tanzu).
	Over time, it has evolved and was marketed into a set of libraries that help .NET developers build cloud-native applications.
	The libraries became bloated with features that were not widely used, while their Spring-based architecture and conventions
	have always felt alien to .NET developers.
	Since the introduction of [.NET Aspire](https://learn.microsoft.com/dotnet/aspire/get-started/aspire-overview) and the
	acquisition of VMware by Broadcom, Steeltoe is returning to its original purpose.
	Steeltoe 4 is a major release that brings many improvements and changes to the library.
	The goal of this release is to make Steeltoe better integrated in the .NET ecosystem in a more developer-friendly way, compatible
	with the latest versions of .NET and third-party libraries/products, and to improve the overall quality of the library.
	This document provides an overview of the changes in Steeltoe 4 and the impact on existing applications.
	Steeltoe 4 requires .NET 8 or higher.
	The Steeltoe project began all the way back in 2016 (before .NET Core 1.0.0 was released) at the request of enterprises experiencing great success with their [Spring Cloud](https://spring.io/cloud) powered applications on [Cloud Foundry](https://www.cloudfoundry.org/).
	These organizations were looking for similar outcomes (such as reduced developer toil, easy-to-implement observability, scaling and resiliency) for their .NET applications.
	Rather than starting from scratch, Steeltoe took the approach of building clients for Spring Cloud services and porting code from Spring as needed, adapting the codebase to fit with .NET while trying to stay close to the Spring origins.
	In order to deliver higher-level Spring project features, Steeltoe libraries grew rapidly with building-block components matching the architecture and conventions in Spring.
	All previous versions of Steeltoe had capability and feature expansion as the main goals, with moderate regard for how well Steeltoe "blends in" with the greater .NET ecosystem.
	As a result, the project hasn't really felt as .NET-native as we'd like it to be.
	Add to that everything that's changed in the .NET ecosystem since .NET Core 1.0 and you can see why it was time for a major overhaul.
	Steeltoe 4 is a major release that brings many improvements and changes with the following goals:
	Steeltoe is well integrated in the .NET ecosystem from the perspective of .NET developers
	Extensibility points are clearly documented and easy to understand
	Steeltoe is compatible with the latest versions of .NET and third-party libraries/products
	Steeltoe is maintainable, well-documented, consistent and contains high quality code
	The introduction of [.NET Aspire](https://learn.microsoft.com/dotnet/aspire/get-started/aspire-overview) and the acquisition of VMware by Broadcom
	underscored that this is the time to refocus on Steeltoe’s core goals and re-evaluate how the desired outcomes are achieved.
	This document provides an overview of the changes in Steeltoe 4 and the impact on existing applications.
	Steeltoe 4 requires .NET 8 or higher.