[mbeddr.doc] Improve copying of images into the doc generation folder

[wsafonov]

Currently images get copied into the target generation folder for the documentation (doc_gen) based on the DocumentConfig settings independent of the actual usage in the document. This leads to the situation that images stored in shared documentation solutions are copied into languages with documentation where those images are not required at all. This can unnecessary increase the size of the packaged languages.

Another related issue: images/resources from the additional paths in the DocumentConfig are currently not copied at all into the target generation folder.