[diff] An app that can accept two documents and compare them

As an author, I would like to compare two documents with the same diffing tools we have for localization.

Start by locating the existing localization diffing tools and understanding how they receive and compare documents. Clarify the app flow for accepting two documents and define the expected comparison result before implementation; done means the two documents can be compared using those tools.

Written by the indexing model from the issue text.