Offer MacOS and Windows shared runners for Gold on GitLab.com

Our CI/CD supports more than Linux, but we only offer shared runners on Linux, which leads people to conclude that that's all we support. Along with improved documentation and marketing materials, we should consider offering MacOS and Windows shared runners. Since the cost structure is higher, I suggest we only make them available to Gold subscribers. Alternatively, we could offer them as a paid add-on, charged per 1,000 minutes or so.

/cc @bikebilly