Point to documentation about metadata from doc/sensitive_documents/graphics
Originally created by @intrigeri on #7368 (Redmine)
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 (closed)