I have a new site that I’m setting up. I just added the modest theme, discourse-docs plugin, and the discourse docs card filter component. In the example on the component page there should be fields where I can add which categories or tags should have cards on the docs page. I have no such entry fields.
Enough time has passed that you’ve probably by now either solved your question, or moved on. But for the benefit of future searchers, the answer is that this is a misunderstanding of how the “Card Filter” theme component works. All it does is make “cards” for tags that are already selected for display in the Docs plugin settings. Docs plugin settings look like this: