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.
Sure thing. Can you remind me where this info comes from? When I do release notes I check the release page and the PRs in the documentation repo, but I guess Insights has its own repo that I should also be checking? Or is there a quick way to check completed sprints (I think there's a way to do it but I don't remember it off the top of my head)?