Store design fine-tuning

Outstanding design tasks for store:
- [ ] Create hero banner image for [each collection](https://store.cncf.io/collections). See [the Phippy category](https://store.cncf.io/collections/phippy-friends) as an example. Make sure it works for both Desktop and mobile (which shows only the left-half of the image).
- [ ] Maybe another image for the homepage hero carousel? We could also consider dropping the white rectangle behind the text if we can find images that keep it accessible.