Merge pull request #451 from w3c/profile-guidance-doc

I'm merging this alone because I think #417 honestly captures the current state of discussion on the intro.
Also, about the many other (non-intro) edits (1) they're mostly of editorial/technical nature; (2) it would be really good if they're approved before the call tonight; (3) I'm not sure @rob-metalinkage  and @kcoyle  will have the time to review all these non-intro commits before then. I hope it's alright for both of you!