Developer documentation for adding support for new CloudQuery platforms in the workspace builder

[vaterlaus]

With the new CloudQuery indexer it's relatively easy to add support for new platforms in the workspace builder (at least for platforms that are supported by CQ). But the process for doing this is still mainly in my (RobV) head, so it would be good to have some documentation for how to do this so other developers can more easily add new platforms. This mainly entails documenting the new PlatformHandler class where the platform-specific logic has been refactored and also the process for creating the platform-specific info that drives how the platform's config file is generated.