[docs] fix sdf guide link, mention msdf
[docs] fix sdf guide link, mention msdf

wizgrav/aframe,fernandojsg/aframe,weddingdj/aframe,ngokevin/aframe,aframevr/aframe,googlecreativelab/aframe,jaxmolina/ar,weddingdj/aframe,ngokevin/aframe,dmarcos/aframe,RSpace/aframe,fernandojsg/aframe,chenzlabs/aframe,RSpace/aframe,aframevr/aframe,darkwing/aframe,mkungla/aframe,mkungla/aframe,darkwing/aframe,dmarcos/aframe,jaxmolina/ar,googlecreativelab/aframe,wizgrav/aframe,chenzlabs/aframe