Fix doc-diff

Created by: ooreilly

As explained in #142, doc-diff crashed when trying to run it on certain modules. This PR fixes the problem. doc diff now uses the same function as make_all_docs for temporarily building the documentation.

As stated in #144, this tool generates html reports whenever there is a mismatch found between any new and old documentation. To see it in action, make a temporary change to any of your already generated docs, e.g., graph/Hypercube.md.