Add "cited by" feature

[allison-truhlar]

Add a new "cited by" feature for all projects with an associated DOI. Possible plan:

• Use a GitHub action to look up citations of each provided DOI on a regular schedule, maybe once per month. Use this information to update the project page with a count and/or list of who has cited the software. Possibly send an email to the project maintainer with these data, to encourage them to look at their project page.
• Use Semantic Scholar API