OpenAPI improvements for minimal APIs sample code

[Rick-Anderson]

Write sample in https://github.com/dotnet/AspNetCore.Docs.Samples/tree/main/fundamentals/minimal-apis/samples/openApi (create folder openApi)

OpenAPI improvements for minimal APIs

[Rick-Anderson]

Looks like it's already done in OpenAPI support in minimal APIs - ASP.NET Core but we should verify there isn't anything new to add.