Release notes 12-9

@mhoeber @lamagnifica @catong Please review.

@clintonb Since you're release lord (should that be capitalized?) this week, you get a sneak peek at the release notes. You don't have to do a thorough review, but feel free to read through them and let me know if anything seems amiss.
I set up additional subtasks and add them to the release notes task so that they don't get forgotten. I don't know why, probably because they seem to be Insights-related rather than release notes-related, but they get assigned to me rather than to whoever is writing the release notes. And therefore they get missed.
Sure, edit at will, that's why I requested a 2nd review opportunity.