Document prepare command

[bennypowers]

Since yarn prepare is reserved, the prepare command should be documented at https://yarnpkg.com/en/docs/cli/prepare

[bennypowers]

Documentation doesn't have to be at that specific URL, but it must be documented that users can't define their own prepare scripts

[Haroenv]

Feel free to do a PR! info should be added in prepare.md here like the other ones.

Did this land in Yarn stable? I don’t think there’s a release yet