Alternate metadata format for tutorials to support third-party sites

[jonathansick]

The current metadata standard for tutorials involves a specific format in the notebook content. Keywords, authorships, and descriptions are extracted heuristically from the content. For third-party tutorial sites, we can't impose this specific format. Instead, we should have a way for authors to define Learn Astropy metadata without affecting their notebook's content. Besides the schema for this metadata, the main question is the format (notebook metadata? YAML sidecar file that's hosted on the website?).