Update index.md

[qinjunj]

Update index.md page

• Change the demo at the beginning (Video link changed, but kept the pretty trailer image) - use ZenGarden or FS
• Move Getting started page to the beginning
• Use IISWC paper
• Update contributors (same as listed in illixr repo)
  - [ ] Update talk, talk video and slides
  - [ ] Change OpenXR demo
  Pointing visitors to the consortium page for talk, video, and slides.