Support generating pretty diff-documents on pull requests

[chrisfenner]

It might aid in review processes to have a flavor of the action that generates a "PDF diff-doc" with the removed content in red, added content in green. Example: https://github.com/davidar/pandiff

This action could be automatically run on pull requests, giving reviewers a chance to easily see changes to large documents. It could also be used to prepare external review request materials.