I did not find a common / preferred way to do this.
For D3, there is a paper I can cite, but for Observable there is none as far as I know (a shame, it could be worth one!): D³ Data-Driven Documents | IEEE Journals & Magazine | IEEE Xplore
So this is a question mostly towards the creators:
How would you like Observable to be cited?
Could you provide this information (e.g., as bibtex) somewhere on your page?