Org-mode is extensive, but each constituent part is not necessarily complex. You can also use use emacs to edit markdown in a quasi-org-mode style. Meaning you

Org-mode is extensive, but each constituent part is not necessarily complex. You can also use use emacs to edit markdown in a quasi-org-mode style. Meaning you can collapse headings globally with shift-tab (which cycles through all hidden, outline visible, all visible), or locally with tab (which similarly cycles through all hidden, outline, all visible). Edit: looks like markdown-mode also supports wiki-links between files. That should give you more or less what you want.