Temporarily disable building docs on AppVeyor
Temporarily disable building docs on AppVeyor

This will get us back to green after the Core 2.0 change, but
obviously we want to go back to building the docs in CI.

iantalarico/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,evildour/google-cloud-dotnet,googleapis/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,evildour/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/gcloud-dotnet,googleapis/google-cloud-dotnet,chrisdunelm/gcloud-dotnet,googleapis/google-cloud-dotnet,benwulfe/google-cloud-dotnet,iantalarico/google-cloud-dotnet,jskeet/google-cloud-dotnet,evildour/google-cloud-dotnet,iantalarico/google-cloud-dotnet,benwulfe/google-cloud-dotnet,benwulfe/google-cloud-dotnet,jskeet/google-cloud-dotnet,chrisdunelm/google-cloud-dotnet,jskeet/google-cloud-dotnet