Improve BioPub site for demoing open groups

So that Heather can show a working example of open groups, we need to improve https://biopub.hypothes.is/ so that it looks and functions like an actual publisher site. Right now, there is a single page with a paragraph of text with lots of test annotations. To make this a workable demo site, we need to:

- Have at least 3 full articles living on separate pages
- Change the front page so that it looks more like the home page of a publisher site
- Add annotations which look and function as real contributions

## Adding new articles
- [ ] Find at least 3 openly available scholarly articles that we can show in full or in part on the BioPub site
- [ ] Create a page for each article and add the content

## Updating the front page
- [ ] Remove Arti's abstract and replace with titles + excerpts of our hosted articles
- [ ] Remove test annotations

## Adding annotations
- [ ] For each article, create 2-3 annotations that look authentic (rather than test annotations).
- [ ] Have a different person post each annotation, and reply to each other's annotations.

*Note:* Heather has mentioned that she does not feel comfortable sharing this site with others until we have completed https://github.com/hypothesis/product-backlog/issues/575. However, the work mentioned in this issue will still be useful, as it can be used by Heather & other Hypothesis team members for demo purposes (because we know enough to not annotate within the restricted group).