A family of Microsoft word processing software products for creating web, email, and print documents.
I checked again, and you actually can assign "Show or Hide the Document Map Pane" to the QAT. If I do this, I can use the assigned button to open up the document map. However, when I close the document and then open it again, the "thumbnails" pane is displayed (so, some setting is telling Word to use "thumbnails" as default).