Meta Data for Discourse OneBox

Coda published a very nice OKR template today via Product Hunt. Interestingly, the ProductHunt link shows up as a gorgeous OneBox here on Coda’s Discourse:

Interestingly the document itself also is gorgeous when linked via a Dicourse OneBox:

However when I copy the document and set it so allow sharing, it appears very differently in the OneBox

So my question is:

What determines the metadata, thumbnails, and other info when a Coda doc is referenced in Discourse? How can I design this to be appealing if I plan (as indeed I do) to use my own Discourse instance to showcase my Coda documents?