github.io document diffing

It might be useful to have a documentation diffing tool that would allow someone who has built documentation for release to compare docs to the latest docs on github.io.

The rough process might look like:
1. Run docs generation pipeline
1. Docs generation pipeline generates current docs
1. Docs generation pipeline pulls current docs from blob storage
1. Docs generation pipeline creates 2 branches in the repo (e.g. "sdk/docs-diff") and puts current docs (from blob storage) in one branch and built docs in another branch
1. Docs generation pipeline creates a PR from the built docs branch into the current branch
1. Link to diff is provided

This would provide a full github diffing experience, though certain things would be missing (like synchronizing changes pushed to branches or merging of the PR)

Opened w/ ideas from @scbedd after speaking w/ @gilbertw