Add discussion of collapsible/expandable text to markdown article


#1

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>