Add first content & technical advances

[SilasBerger]

• Replace demo material with first real content
• Implement marker system: select elements based on filename
• Add a draft site for local development
• Improve tiles and YouTube component
• Add support for Jest, work on tests for builder
• Fix Sass issue
• Implement workaround for @site/ imports in IntelliJ MDX plugin
• General housekeeping
• Add documentation
• CI improvements
  • Build all sites with matrix strategy, deploy to web host (main)
  • Run checks (main, feature/*)