The markdown article at https://blog.ghost.org/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>