Remove `start.sh` and switch to `sg` to run dev environment

Created by: mrnugget

2 weeks ago, in #24988, we announced that we'll remove start.sh in 2 weeks. Time's up.

This the majority of start.sh but leaves a message in it saying that we switched to sg and where to find documentation.

It also removes all the shell scripts that start.sh used to boot up the dev environment that we don't need anymore, either because they're built into sg (such as the watchman configuration, wrapper, helper scripts) or because they're now in sg.config.yaml (where we'll step-by-step reduce them by building more into sg).