Link the Figma Document/Design language/Design standards in more places in the contributor guide.

[fallible-algebra]

As Bevy UI gets more capable (thanks BSN & Feathers!) we should document standards for examples & the editor in more areas in the contribution guide.

Right now Figma is what the editor UI has been sketched out in, so it'd be a good candidate for a thing to link to for this.

[mockersf]

I don't know sigma enough to answer, but is there a way to lock a revision? Links saying "here is the standard for example: ..." should be locked and approved