Clarify vsts-npm-auth only required when adding new packages

[Copilot]

The CONTRIBUTING.md incorrectly implied that vsts-npm-auth authentication was required for initial setup. Authentication is only needed when adding new packages to the Azure DevOps feeds, not for installing existing dependencies.

Changes:

• Building section: Removed vsts-npm-auth from initial setup steps, added note clarifying it's only needed when adding new packages
• Updating NPM packages section: Enhanced with explicit note that authentication is only required for adding packages, not installing existing dependencies

This allows new contributors to build the project without Azure DevOps authentication, reducing friction in the onboarding process.

Original prompt

update contributing.md to mention that vsts-npm-auth is only needed if adding new packages to the feeds

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