docs: Document using an alternate clone URL for repos

Created by: sqs

Some sites put an HTTPS or SSH proxy in front of their code host to reduce load. Some sites also use a service like AWS CodeCommit to do the same. In these cases, the repos still should be treated as being repos on the original code host, not the proxy site.

For example, I have a GitHub repo github.com/foo/bar. I want Sourcegraph to clone it using the URL https://cloneproxy.example.com/foo/bar.git. But I still want the "Go to GitHub repository" button, etc., to take me to https://github.com/foo/bar.

• Document workaround of using ssh_config or gitconfig's insteadOf
• Support this as a first-class feature (not scheduled yet)

The actual code work is not prioritized, this is just a docs change.