Clarify dotnet new NuGet source option

[Copilot]

The --add-source|--nuget-source docs implied the option adds a source to the configured NuGet source list. The option only adds a source for the current dotnet new operation.

• Clarified source behavior

  • Updated dotnet new install, dotnet new update, and dotnet new details to state that --nuget-source is used in addition to configured sources for the command only.
  • Explicitly notes that the option doesn't modify NuGet configuration.

• Cleaned related command docs

  • Corrected the dotnet new update option heading to include --nuget-source.
  • Fixed copied wording that referenced dotnet new install from the dotnet new update page.

• Fixes The --nuget-source option description could be misleading #55188

[Copilot]

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