Add PSM entry to example nuget.config

[baronfel]

Added package source mapping for specific package sources.

I often reference this example config and was surprised to not see PSM config in it.

[learn-build-service-prod]

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[learn-build-service-prod]

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[Copilot]

Pull Request Overview

This PR adds Package Source Mapping (PSM) configuration to the example nuget.config file, demonstrating how to enforce that certain packages only come from specific package sources.

Key Changes

• Added a packageSourceMapping section to the example configuration showing how to map package patterns to specific sources
• Demonstrates mapping ES.* packages to "MyRepo - ES" source and all other packages to the official NuGet source

[Copilot]

The indentation of the packageSourceMapping child elements is inconsistent with the rest of the example configuration file. The <packageSource> and <package> elements should use 4 spaces of indentation to match the indentation style used in other sections like <config> and <packageSources>. Currently, these lines appear to use 2 spaces instead of 4.

Suggested change

	<packageSource key="MyRepo - ES">
	<package pattern="ES.*" />
	</packageSource>
	<packageSource key="NuGet official package source">
	<package pattern="*" />
	</packageSource>
	<packageSource key="MyRepo - ES">
	<package pattern="ES.*" />
	</packageSource>
	<packageSource key="NuGet official package source">
	<package pattern="*" />
	</packageSource>