Final cleanup

[DaveTrost]

• Add text to the play/pause button in the background section. "Animate it"/"Close Animation"
• remove subcategories from the skills section until it coordinates with the rest of the site
• Refactor animation and Intersection Observer code
• make adjustments based on the test builds
  • "content" is not animatable according to the spec. Use Javascript instead
• check animation timing on multiple device sizes