fix(Article): Do not require authors and title

After using the `Article` type in Encoda extensively, it seems inappropriate to have these as required. The `Article` type is often used as a container for content. In Encoda, we often have to use an empty author list and an empty string, or 'Untitled' for title. Most of the time these requirements feel uncessary.

schema/Article.schema.yaml

@@ -5,9 +5,3 @@ role: primary
 status: unstable
 category: metadata
 description: An article, including news and scholarly articles.
-
-required:
-  # See Google's list of required properties at
-  # https://developers.google.com/search/docs/data-types/article#article_types
-  - authors
-  - title