Add reference thumbnail images for the spec docs

[crydalch]

Per TSC discussion on 7/16, it would be good to have some visual reference to the MaterialX speculation docs, for artists or engineers implementing the spec. Just to help solidify some expectations on how the various shaders and nodes ought to look/behave (currently, the best visual representation is often a PR).

Perhaps someday those can be replaced with live, rendered widgets, but for now we'll just tackle small images inline with the markdown docs.