Display TOC on top of each page

[jmuheim]

Like in Wikipedia, we would love to have a small TOC on top of each page with in-page links:

This is an important step to:

• Raise users' interest when arriving on a page (at the time being, users leave pages pretty quickly)
• Make users aware that there is an "You could also be interested in..." section at the end of some pages (which we hope would keep them on the website instead of leaving)

@backflip I think you're the man for this job. Could you do this during the upcoming 2-3 months? ❤️