Add how-to for using workspaces in campaign spec
Created by: mrnugget
This adds a how-to that explains how to use the new workspaces
feature (added in https://github.com/sourcegraph/src-cli/pull/442) in campaign specs.
Click here for the preview.
I know that this kinda blurs the line between how-to and tutorial, but I have to say that I found it hard to explain how to use this feature without making it more concrete. And since we have our campaign spec YAML reference, I think it's okay to get a bit more concrete here.