Light Freshness Edit: ASP.NET - Startup, gRPC, YARP, SPA

[GitHubber17]

This PR contains light freshness updates to the following articles:

• App startup in ASP.NET Core
• Overview for gRPC on .NET
• gRPC services with C#
• Overview of Single Page Apps (SPAs) in ASP.NET Core (and an include file)
• Use React with ASP.NET Core
• Getting started with YARP
• YARP Configuration Files

For details, see ADO request 568764.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/client-side/spa/intro.md	customer intent: As an ASP.NET developer, I want to get an overview of SPAs in ASP.NET Core, so I can determine the appropriate tutorial to create apps.
aspnetcore/client-side/spa/react.md	Use React with ASP.NET Core
aspnetcore/fundamentals/servers/yarp/config-files.md	YARP Configuration Files
aspnetcore/fundamentals/servers/yarp/getting-started.md	Get Started with YARP
aspnetcore/fundamentals/servers/yarp/header-routing.md	YARP Header Based Routing
aspnetcore/fundamentals/servers/yarp/https-tls.md	aspnetcore/fundamentals/servers/yarp/https-tls
aspnetcore/fundamentals/startup.md	customer intent: As an ASP.NET developer, I want to explore the Startup class in ASP.NET Core, so I can configure services and the request pipeline for my apps.
aspnetcore/grpc/basics.md	gRPC Services with C#
aspnetcore/grpc/index.md	customer intent: As an ASP.NET developer, I want to explore gRPC services with Kestrel server and the ASP.NET Core stack, so I can use gRPC in my project.