Get the thumbnail info for a document in the default workspace.

• By default, returns thumbnail info for the element with the most-recently generated image. If you pinned an element for the document thumbnail, that element will always be used for the document-level thumbnail, if it exists in the workspace.
• The default workspace may vary by user; the image served depends on the signed-in user.
• See also: Tech tip on how to change a document thumbnail in onshape