Add ontobot github action to show diffs on PRs

[cmungall]

This will make it easier to see the content of changes, as it's hard to read text-level diffs

simple robot diff is fine, no need for entailment diffs (since we don't get classification of OPs in OWL)

We think it would be better if this is triggered by the magic hashtag (#gogoeditdiff for other ontologies) (need to also document this)