Add more example sample assets

[madjin]

Action items

• Some kind of sample scene for an extension (.blend / unitypackage / etc)
• Sample object with the nodes of current proposed spec
• Link to current implementation somewhere

Physics ones have examples, maybe we also point to live demos via different implementations (web / native)

Missing samples / assets

• KHR_audio
  • cross-reference hubs blender add-on
• OMI_link
  • cross-reference hubs blender add-on
  • cross-reference oncyber blender docs (name scheme for portals)
• OMI_personality
  • @antpb has demo
• OMI_seat
  • has examples, could use some links to demos
• OMI_spawn_point
  • has examples, could use some links to demos