Product investigation: which formats do we want the Preferred Source button to appear on?

This button was put out in a hurry. While we got a large enough bang for our buck by filtering out a handful of [weird and wonderful ArticleDesigns](https://github.com/guardian/dotcom-rendering/blob/main/dotcom-rendering/src/preferredSource.ts), we probably want to ask whether there are other designs we want to support or exclude.

For instance:
- Should this button appear on LiveBlogs/DeadBlogs, which are usually quite news-y and high traffic?
- Should we exclude this button from crosswords, which are definitely not news-y
- Should we add this button to Opinion pieces, which are often news-adjacent but less likely to appear in news-related searches