Display the headings of articles and chapters

[julien-deramond]

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Add something like the following on the page as done by Google Docs:

Motivation and context

When the content is long, it can be really useful to be able to jump to a specific section (especially if you've already read the content).
Can also be useful for fast readers who analyze the content structure before reading it.