Notebook side-navigation based on markdown headings

Created by: benvenker

Experiment showing nesting based on markdown heading levels: https://www.figma.com/file/qy30bGoHzSI0SDDw9rnIuL/Search-Notebook-experiments?node-id=202%3A11149

• Should persist when scrolling on the left hand side (too much going on right hand side with buttons)
• Should scroll to selected section when clicked

We'll get proper design on this at some point. Posting experiment here to preemptively unblock initial implementation.