Add headers to dataset pages to enable Google Dataset Search

I discovered in making the YAML headers to document our datasets that it's only one more step to create a dataset header schema (developer documentation [here](https://developers.google.com/search/docs/data-types/dataset)), so that our datasets show up in Google Dataset Search.

I plan to implement this after YAML is set up so that our datasets will be searchable and findable on Google Dataset Search!