[self-hosted] Document how to use Gitpod within an http_proxy environment.

[metcalfc]

This started with openvsx. Folks were trying to figure this out through experimentation. They started by setting https_proxy for openvsx-proxy and that allowed searching of extensions. They then set https_proxy in the workspace settings in VS Code and that seemed to work.

We should document what the right way is to do this.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.