Update image checking functionality in liferay-doc-utils. Missing images, images with incorrect paths, extra images should all be flagged.
Currently, the image checker (the one on which dist depends) is checking that a file exists in /images/ with the same name, but it isn't checking that the image file's relative path is correct. The importer doesn't have an issue with this because it just adds all image files as attachments and doesn't use the relative path information; but the article images aren't rendered for the Markdown files in GitHub.
This issue must be fixed so we can ensure that articles' images render correctly when viewed (on Github, for example).