Point to documentation about metadata from doc/sensitive_documents/graphics

It would be good if our documentation about creating documents (especially, graphic ones) pointed to the one (if it exists already) about risks of metadata in such files, and (either directly, or transitively) to the MAT documentation.

Feature Branch: doc/7368-warning_metadata

Parent Task: #8416

Original created by @intrigeri on 7368 (Redmine)