docs(yarn): docs to clarify expected setup for yarn 2+

[Cynocracy]

This documents what the local .yarnrc.yml should look like, and that it CANNOT contain a top level reference to NPM_TOKEN (unlike .npmrc)

Changes

Doc-only change

Context

Clarify yarn 2+ private registry support https://github.com/renovatebot/renovate/issues/14756

Documentation (please check one with an [x])

• [x] I have updated the documentation, or
• [ ] No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• [x] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [ ] Both unit tests + ran on a real repository