In cases like T135315: Content service shifts wrong lead paragraph to the top in certain cases [1] it would be helpful to have an easy way to identify where we've shifted the lead paragraph from the original Parsoid HTML. We should add a class ('shifted' or something, doesn't matter as long as it's unique) to lead paragraphs that we've shifted via relocateFirstParagraph.js.

We should also think generally about where it might be useful to mark our transformations for reference later.

[1] see also T136964: Pre-generate/purge mobile-sections endpoints to fix page links inside image captions