Link rewriter step

[pydsigner]

Use custody information to track how file names change, then rewrite links to those files accordingly in HTML, CSS, and perhaps JS.

[pydsigner]

Big issue to solve— how do you pick between different downstreams, for example between thumbnails and new file types? Distinguishing between template inputs like a Jinja HTML and the real input like an MD isn't hard because you start with the MD file in the link, but telling between a Jinja index of those MD files and the rendered MD will require a heuristic as well.