Update react-compiler docs

This upstreams some of the [introduction
post](reactwg/react-compiler#5) into the
docs since this knowledge is quite fundamental to understanding the
compiler, and it's confusing to not have it linked in the canonical
source for the docs. It also adds more information about
[debugging](reactwg/react-compiler#7) and
reorganizes a few sections to make things clearer

ghstack-source-id: ef32fc7
Pull Request resolved: #6895

Author: poteto