[Wishlist]: Separate long doc into tabs in the editor

As a doc gets too long, it's hard to find where you want to edit, and takes a lot of time scrolling.

It'd be good if a long doc can be separated --- behind the scene --- into tabbed sub-docs, then automatically assembled into a whole doc upon rendering.

With that feature, it'd be also much easier for users if relocation of a section is needed --- just drag the tab and drop it between other tabs.

Start by locating the editor entry point and the rendering path for documents; the issue does not name files or tests. Clarify how long documents become tabbed sub-documents, how tabs are reordered, and how the pieces are assembled on rendering. Done would include working tab navigation, drag-and-drop relocation, and complete rendered output.

Written by the indexing model from the issue text.