Add npm Package Registry testing documentation

What does this MR do?

Adds documentation for testing npm Package Registry features with GDK.

Why is this needed?

When developers onboard to Package Registry work and follow the npm Package Registry authentication documentation, they may create an .npmrc file in the GitLab repository root. This breaks the GDK because it interferes with GitLab's own npm dependencies.

This documentation clarifies that .npmrc should be created in a separate test project directory outside the GDK installation.

Related issues

Addresses a common onboarding issue for Package Registry team members.

Checklist

• Documentation added to doc/howto/package.md
• Follows existing structure and formatting of the file