Add instructions for private instance account authentication

Created by: jjinnii

We have received customer feedback that it is confusing to know how to set up a private Sourcegraph instance to the VS Code extension. There is an extra step that private instance users have to change the base URL to their private instance, that is currently hidden under "Extension Settings".

With the interest from our customers (yay!), I think it's worth making a separate section for "private instance" authentication, even if it's just a single additional step.