Introduce link check on pull requests

[StevenMaude]

Occasionally, broken links get introduced into the site, which we detect via a scheduled link check sometime they are included. It would be preferable to not introduce them at all.

To avoid this, we could check the files modified in pull request, in addition to having a regular scheduled link check.

If successful, we could also use this approach for other sites that we maintain.

Considerations:

• The Lychee GitHub Action that we use doesn't have a built-in feature for this to dete; though there are scripted workarounds, see Checking only links from modified files in a Pull Request? lycheeverse/lychee-action#17
• Sometimes links aren't broken, but simply blocked because the site blocks the GitHub Actions runner and/or the Lychee client. The fix then is to add the link to the .lycheeignore, which is less obvious. Might this be extra confusion for pull request authors?