Renaming image files along with document renaming

As noted at https://github.com/opendatakit/docs/pull/642#issuecomment-380251532, we should rename image files along with the documents.

Start by reviewing the discussion linked from pull request 642, especially the referenced issue comment, then inspect how document renames are represented in this Sphinx documentation repository. The work is done when image files are renamed alongside their corresponding documents and all affected references remain consistent.

Written by the indexing model from the issue text.