Add docs story on using proprietary extensions

While we have docs on how to use a proprietary extension (one that's not in openvsx.org), it is rather buried under other info and hard to find (see https://www.eclipse.org/che/docs/stable/administration-guide/extensions-for-microsoft-visual-studio-code-open-source/#adding-or-removing-extensions-in-the-embedded-open-vsx-registry-instance)

Document a Using a proprietary extension story.