I've been looking for a text editor that automatically (or at least makes it easy to-) make some sort of navigation. Adobe Reader has this functionality with its bookmark system:

Right now, though, I'm using Word 2007. For each section, I go Insert > Bookmark, highlight the text, copy/paste the text as the link information, and it appears at the top of the document. I've made a macro to add bookmarks easier, but it's still pretty awful, and the bookmarks are still at the top of the page (rather than in the sidebar, where it's always accessible)

Honestly, I would just prefer to write it in a PDF like in that screenshot. But any text editor with this type of functionality would work. It just needs basic formatting options, bold/font size, underline, images, maybe tables.