Add history/context to performance-scoring docs

In the '6.0 Whats new' post we have some useful text and framing that shoudl be added to https://web.dev/performance-scoring/

* 🔒 https://goto.google.com/mxggu (at the very bottom) has good text around control points and score curve design
* the shipped "Performance score update" section has a good reasoning behind the v6 changes that many would fine useful.

let's incorporate those!