With some well-timed pestering, I’ve managed to convince the Discourse team to enable an automatic Table of Contents for posts which use markdown headings.
To enable it, you need to hit the 
icon in the composer bar, and then Insert table of contents:
This inserts the following code to your post (it can go anywhere), which activates the functionality:
<div data-theme-toc="true"> </div>
People with access to the CoPs can see it in action for @david.vink’s post here:
https://ehealthforum.nz/t/meetings-minutes-and-agendas/29916/68