docs: Document .NET 8 self-contained deployment default change and add troubleshooting

[Copilot]

In .NET 8, specifying -r <RID> no longer implies --self-contained — apps now publish framework-dependent by default even with a RID. This silent behavioral change from .NET 7 has caused widespread confusion (16k-view SO question).

Changes to docs/core/deploying/index.md

• Breaking change callout — Added [!IMPORTANT] alert at the top of the Self-contained deployment section, explicitly stating that -r <RID> alone no longer produces a self-contained app in .NET 8+:

  # .NET 7 behavior (implicit self-contained)
  dotnet publish -r win-x64
  
  # .NET 8+ required form
  dotnet publish -r win-x64 --self-contained

• Troubleshooting section — Added ## Troubleshoot self-contained deployment before ## See also with:

  • App prompts for .NET runtime installation — root cause explanation + fix for both CLI and Visual Studio
  • Verify that publish output is self-contained — tells readers to check for coreclr.dll / libcoreclr.so / libcoreclr.dylib in the publish folder; includes project-file fix via <SelfContained>true</SelfContained>

• See also — Added link to the Runtime-specific apps no longer self-contained breaking change article

[gewarren]

This should probably go under the self-contained section, if it's needed.