The markdown article at The Ultimate Guide to Writing & Publishing with Markdown is fantastic, but even in the advanced section it fails to discuss collapsible/expandable text. This seems to be currently possible in markdown (e.g. the below text seems to work for me when I try it). Please consider adding this to the advanced section of that article, and any related abilities too!
<details>
<summary>Your header here! (Click to expand)</summary>
Your content here...</br>
(markup only where supported)</br>
more content here...</br>
</details>