Feature/improved diff utils and docs

[essweine]

This PR adds a few helper functions to the diff utilities and documents potential usage in the example app. Necessary changes to that app are on this branch: https://github.com/sartography/spiff-example-cli/tree/feature/workflow-migration

[essweine]

Note that some links in the new docs won't work until the branch with changes in the example repo gets merged.