devops: adds jekyll build action for Ruby 2.7, 3

[mattxwang]

This PR adds a GitHub Action to build the site on Ruby 2.7 and 3. Part of #92.

[mattxwang]

@ctrueden any thoughts on the PR? Once this is merged, I can address the other portions of #92!

[ctrueden]

Thanks a lot @mattxwang, and sorry for the delay. It's been a super busy week.

It does seem very likely at this point that we will migrate from vanilla GitHub Pages to deplyoment via GitHub Actions, because there is at least one Jekyll plugin (jekyll-scholar) that I'd love to enhance and use. Right now we use citation-js to populate citations specified by DOI, but it has to happen on the client side upon page load, which feels kind of rude to whatever infrastructure is serving up the citations. It would be fewer requests, and also faster for page load times, to just bake in the citations at site build time. But to do that, we need to enhance the Jekyll build process somehow. The jekyll-scholar plugin looked like a good candidate for a starting point to enhance to have this capability. Let me know if you have a better idea.

[mattxwang]

Sounds good to me! I can spin up the action to deploy from GitHub Actions (to GitHub Pages) if you'd like! The only thing is that someone will have to add a repo access key to do the actual deploying.

(an alternative would be using Netlify, as discussed earlier, which also has the benefit of deploy previews - let me know what you think!)