docdiff: highlights unchanged tables, not actual changes

Example PR preview: https://plone6--1679.org.readthedocs.build/install/containers/images/backend.html#arbitrary-user-and-persistent-data

When I use `d` to toggle between docdiff views, the changes from the PR do not appear as a diff, but instead all tables in the page display diff highlighting even though there were no changes to them.