Hey @henri.xvz,
The example you linked to is done with the markdown card, typing “/markdown” in the editor will bring up a markdown editor for you. There’s an example here of how to add footnotes in markdown:
Example:
Here's a simple footnote,[^1] and here's a longer one.[^bignote]
[^1]: This is the first footnote.
[^bignote]: Here's one with multiple paragraphs and code.
Indent paragraphs to include them in the footnote.