Enable annotations on doc previews

[debadair]

Now that we have previews, @kellyemurphy pointed out that it would be nice to enable people to comment directly from the rendered docs instead of from the GitHub diff. Something like http://annotatorjs.org/ might be a way to enable that.

It's important that all comments are fed back into the PR in GitHub, though. (Trying to keep track of and reconcile different comment streams is crazy-making.)

Relates to ##336