Add AddCSharpApp and CSharpAppResource documentation to app-host-overview.md

[Copilot]

Fixes #5315

This PR updates the Aspire orchestration overview documentation to include coverage of AddCSharpApp and CSharpAppResource, which were added in Aspire 9.5 to support C# file-based apps and provide an alternative way to add C# projects to the app model without requiring project references.

Changes

Built-in resource types table

Added a new row documenting the AddCSharpApp method and CSharpAppResource type, positioned directly after AddProject to show the relationship between the two approaches.

Enhanced project resources introduction

Updated the introductory paragraph to mention both methods for adding C# projects to the app model, noting that AddCSharpApp provides an alternative that doesn't require a project reference.

New AddCSharpApp documentation section

Added comprehensive documentation explaining:

• How AddCSharpApp differs from AddProject (path-based vs project reference)
• When to use AddCSharpApp (file-based apps from .NET 10, or projects without adding references)
• Code examples showing usage with both .cs files and project directories:

  // Adds a file-based C# app "inventoryservice" from a .cs file.
  var inventoryService = builder.AddCSharpApp("inventoryservice", @"..\InventoryService.cs");
  
  // Adds a C# project "catalogservice" from a project directory.
  var catalogService = builder.AddCSharpApp("catalogservice", @"..\CatalogService");

• Configuration compatibility with AddProject (replicas, environment variables, resource dependencies)
• A note explaining the experimental status and .NET 10 SDK requirement for file-based apps

All changes follow the repository's documentation guidelines and have been validated with markdownlint.

Original prompt

Please work on issue #5315 - updating app-host-overview.md

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/app-host-overview.md	Aspire orchestration overview