Prevent or detect broken links to internal pages

[lbianchi-lbl]

After noticing a broken internal link in one of the pages I've written (#9) I've also looked into a few ways to reduce the chance of this happening in the future:

A) https://github.com/benbalter/jekyll-relative-links: a Jekyll extension that allows using relative paths to Markdown files as a way to specify internal links (i.e. the way it works on GitHub)
B) Add a CI check for broken links, e.g. https://github.com/gjtorikian/html-proofer, using GitHub Actions

Disclaimer:

• I don't have much experience with Jekyll (these are just things I've found during a 10-minute search), so I definitely don't have strong preferences or opinions on how to go about
• If you (and the technical/website folks in particular @mrmundt @cabejackson in particular) think this could be useful, I'd be happy to try and throw together a quick GitHub Actions workflow to run the linkchecker for PRs

[mrmundt]

@lbianchi-lbl - We actually have a linting workflow on the main USRSE/usrse.github.io repository that we can re-use. Link: https://github.com/USRSE/usrse.github.io/blob/main/.github/workflows/linting.yaml