Fixed CI, improved developer docs, and extracted markup to datafiles

[Songtech-0912]

Just some functional changes to get things working:

• GitHub action completely rewritten to support our external plugins that GitHub by default does not support
• Since we are using GitHub actions for Jekyll, there is no need to be constrained to GitHub's dependency versions so:
  • I've bumped the Ruby version to a modern version to make bundler work both locally and on CI, it is in .ruby-version
  • No more bloated github-pages gem! This reduces build time significantly (from over a minute to ~12 seconds) as we don't use the majority of its features anyways
• Improved docs (and new ones for building on windows) because setting Jekyll up properly is way harder than it seems and can take up to an entire morning of troubleshooting, with special notice to contributors for things that may accidentally break builds
• Extracted the homepage's text contents to YAML files in _data/ which should make it much more maintainable
• CI works now!

[Songtech-0912]

Added in the changes you requested :) let me know if there is anything else important.