Link working three.js demos from Servo homepage

[Manishearth]

https://manishearth.net/sand/demos.html has a bunch of links to three.js webxr demos tuned especially for AR (e.g. background meshes are removed).

(They're also published on https://manishearth.github.io/webxr-three/examples/* , which might be a better URL to use)

We should make these the main demo on http://servo.org/hl-home , perhaps with the babylon demos further down since they don't quite work yet

cc @jdm

[paulrouget]

Should we copy them under /hl-home/ or should we just link them to https://manishearth.github.io/webxr-three/examples/*?

[Manishearth]

I think directly linking is fine, I plan to keep those up to date with fixes.