Clarify "presentation based metadata" in tech details page

[azaroth42]

@kshawkin: At http://iiif.io/technical-details/ , under "step 2", you might change "presentation based metadata" to "structural metadata". I think that would be clearer.

@zimeon: I agree that "presentation based metadata" isn't good, it doesn't even really make sense. However, "structural metadata" seems too restrictive: we have technical metadata, structural metadata, and additional metadata for presentation such as labels, rights information, etc..

@kshawkin: Hmm, I see. How about "rendering metadata"?

From: IIIF/api#889