get-started/add-aspire-existing-app for C# missing step to add project references

[DamianEdwards]

https://aspire.dev/get-started/add-aspire-existing-app/?lang=csharp#model-your-resources-in-the-apphost

This is missing the crucial detail that any projects you want the AppHost to orchestrate need to be referenced by the AppHost project, e.g. the API and Web app projects. aspire init will discover projects and prompt which ones to add to the generated AppHost project, but if you need to add more later manually, not knowing this detail will likely lead to folks getting stuck. Without the project references, the marker types required to pass to AddProject<T> aren't generated.

After the "Install hosting packages" and before the "Model your resources in the AppHost" step, there should be a step detailing that C# project resources need to be added as <Project Reference /> items in the AppHost project file, e.g.:

$ ExampleCommerce.AppHost> dotnet add project reference ../Api/ExampleCommerce.Api.csproj
$ ExampleCommerce.AppHost> dotnet add project reference ../Web/ExampleCommerce.Web.csproj
$ ExampleCommerce.AppHost> dotnet add project reference ../OrderProcessor/ExampleCommerce.OrderProcessor.csproj

aspire init willl do this automatically for projects selected when it asks, but it's important the detail is expressly called out.