Document outline should have option to be always-visible

[agoose77]

Presently, DocumentOutline hides itself if there are fewer than two headings in the document. I think we should make this something that users can disable if they wish. Unlike the concept of a dynamic primary sidebar discussed in #458 (hide the primary sidebar if there is no TOC or nav), the document outline feels more persistent and currently disappears / reappears on a per-page basis.