This is a small PR that:
- adds a `fixtures/Gemfile-github-pages`, the minimum gemfile necessary to build the site
- adds a new Actions workflow to build the site with that gemfile
In the future, this should prevent issues like #1074, where we push a change that passes the `jekyll` CI but fails to build on `github-pages` (and its dependencies - which in this case, was an older version of ruby sass).
(to be honest - not super happy with the code duplication, but I also think we'll have different build matrices for the different versions, so I'm fine with this for now)
Part of (but does not close) #1145.
