Add dropdown (accordeon) for markdown sections

[dorn-gerhard]

It would be cool to have markdown sections expandable like a UI accordion (https://uxdesign.cc/ui-cheat-sheet-accordions-3e88f0d4dfee)

some specifications:

• All cells up to the next section (same hierarchy) turn (in)visible.
• All cells appearing in the context menu will have that collapse🔺 / expand button 🔻