Issue # 3494493: Documentation: Document Maintainer Widget

This MR implements the Block Plugin for the Document Widget as suggested by @drumm in the MR: bluecheese!136 (merged)

I believe I asked for all functionality like this to be moved to drupalorg module as a block. The theme needs to remain lightweight and flexible as it is used on multiple sites, and should not have content-type-specific additions like this.

Closes #3494493