[docs] add don's displacement shader example to material docs
[docs] add don's displacement shader example to material docs

ngokevin/aframe,ngokevin/aframe,wizgrav/aframe,RSpace/aframe,wizgrav/aframe,mkungla/aframe,dmarcos/aframe,RSpace/aframe,mkungla/aframe,dmarcos/aframe,chenzlabs/aframe,aframevr/aframe,aframevr/aframe,chenzlabs/aframe