'New React Docs' GitHub Issue is interesting and shines some light on the changes.

Significantly, note that it took a litle bit more than a year from the formulation of this issue to the Beta version. Knowing that there were some holy wars to be fought on the way, I don't think it's too long. Congrats to the team who worked on it!

[1] https://github.com/reactjs/reactjs.org/issues/3308

I had asked Rachel Nabors about that a few weeks ago, and she said she had a post drafted and would put it up post-beta launch.