Integrate migration script into CLI

The migration script https://github.com/icaraps/migration-script generates a Gatsby site with existing documentation repositories from GitHub. It uses https://github.com/AdobeDocs/gatsby-theme-parliament-documentation as template.

Documentation repositories are markdown based and usually have a manifest.json file to describe meta-data like name and side navigation etc. For example https://github.com/AdobeDocs/project-firefly renders https://icaraps.github.io/project-firefly/docs/README/.