docs: add "Add project references" step to C# add-Aspire-to-existing-app guide

[Copilot]

The C# path of the "Aspireify an existing app" guide was missing a critical step: the AppHost project must have <ProjectReference> entries for every C# project it orchestrates, because those references are what generate the Projects.Foo marker types required by AddProject<T>. Without this, users hit confusing build errors after manually expanding beyond what aspire init scaffolded.

Changes

• New ### Add project references section (C# pivot only) inserted between "Install hosting packages" and "Model your resources in the AppHost" in both the English and Japanese docs
  • Explains why references are required (marker type generation)
  • Notes that aspire init handles this automatically for initially selected projects
  • Shows dotnet add reference commands for adding projects later:

    dotnet add reference ../Api/ExampleCommerce.Api.csproj
    dotnet add reference ../Web/ExampleCommerce.Web.csproj
    dotnet add reference ../OrderProcessor/ExampleCommerce.OrderProcessor.csproj

  • Shows the resulting .csproj XML for users who prefer editing the project file directly

Original prompt

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This section details on the original issue you should resolve

<issue_title>get-started/add-aspire-existing-app for C# missing step to add project references</issue_title>
<issue_description>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.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes get-started/add-aspire-existing-app for C# missing step to add project references #470

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[DamianEdwards]

Looks good to me

[Copilot]

Pull request overview

This PR adds a critical missing step to the "Add Aspire to existing app" getting started guide. The C# path was previously missing documentation about the requirement for AppHost projects to have project references to orchestrated C# projects. These references are essential because they trigger the code generation that produces the Projects.* marker types required by AddProject<T>. Without this step documented, users who manually add projects beyond what aspire init scaffolds would encounter confusing build errors.

Changes:

• Added new "Add project references" section to the C# path (both English and Japanese versions)
• Section placed strategically between "Install hosting packages" and "Model your resources"
• Includes explanation of why references are needed (marker type generation), notes that aspire init handles this automatically, and provides both CLI commands and XML examples for manually adding references

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/frontend/src/content/docs/get-started/add-aspire-existing-app.mdx	Adds "Add project references" section with CLI commands and XML examples for C# projects in English
src/frontend/src/content/docs/ja/get-started/add-aspire-existing-app.mdx	Adds "Add project references" section with CLI commands and XML examples for C# projects in Japanese

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[sebastienros]

I think there is a subtle grammatical error here