Update Jekyll setup

This PR should successfully migrate us to a Gem-based JTD setup, without all the clutter from the original repo (see #66 (closed)). Hopefully, this will make the repo less confusing for new contributors, and give us a solid base on which to add new features.

Changes are too numerous to list entirely but:

• I have moved our custom SCSS (e.g., the hero-unit custom CSS class) or HTML (e.g., fontawesome stylesheet) to new files, which do not overlap with JTD's.
• I have pinned our Ruby docker image to 3.3, and committed Gemfile.lock to version control, to avoid breaking changes in Jekyll as seen recently with Ruby 3.4.
  • We can update to 3.4 on the next release of JTD or when the fix is backported into Jekyll.
• General cleanup, including old MD files, and github action workflows, obviously no longer used.

This PR is quite large (all changes had to be done at once) so some things may have broken that I missed. The SCSS warnings are still there, as these are related to the theme authors' SCSS, not ours. I'm sure this will be resolved upstream at some point.