Subdocs - implement image fixing

[Swader]

Subdoc parsing currently does not fix images like it does for main READMEs. This should be implemented, but has some caveats about folder levels - i.e. if a subdoc is two levels deep and the image is one level above, which is one level below root, then it should look for an image one level deep. This means this fix needs to keep track of the path structure of the subdocs.