Fix relative references in SVGs generated by sphinx.ext.graphviz

[Panaetius]

closes #11077

[grahamwaters]

could this help?
https://github.com/grahamwaters/ReferenceMedic

[Panaetius]

could this help? https://github.com/grahamwaters/ReferenceMedic

I don't think it does. It doesn't support svg files at the moment. And it seems to just fix paths for a very specific folder layout, whereas in this case we know exactly from which source document to which target folder relative reference need to be changed in sphinx, and what type of attributes can be affected, so searching svg (XML) with a regex sounds like a bad idea when we can instead specifically look for e.g. href attributes to update and not accidentally update something that's not relevant. Especially since ONLY links/references need to be updated, paths in plain text do no need to be changed.
Also that library only deals with file paths, but we need to fix URLs in general (Like #my-anchor needs to be changed to ../mydoc/subdoc/#my-anchor). The targets can be anywhere in the file structure, essentially, depending on where the source *.rst file was, not in a fixed directory structure.

[AA-Turner]

Thanks @Panaetius!

A