This provides a sidebar buffer which shows a "table of
contents" for an associated outline-mode buffer.  Basically,
this shows you the sections of the outline-mode buffer, but
not the bodies.  This is to help you remember where you are in
a large document.

Simply use M-x outline-toc-mode to toggle activation of the
outline-toc.  Use 'M-x customize-group RET outline-toc RET' to
adapt outline-toc to your needs.

Much of this was originally adapated from David Engster's
excellent minimap.el (https://github.com/dengste/minimap).