Addon-docs: Basic skeleton, UI viewMode handling

[shilman]

Issue: #7101

What I did

This is a skeleton for addon-docs. It introduces a new viewMode called docs that renders a react component, params.docs in the preview iframe. In subsequent PRs that component will get filled in with either the DocsPage ("auto-generated docs") or MDX loaded from a file.

This PR builds on #6382, so it should be reviewed after that PR is merged into next. Just queueing up the work here.

How to test

yarn test
cd examples/official-storybook
yarn storybook 
# view docs tab for Addons|Docs story => "hello docs"
# view docs tab for everything else => "No docs found"

[vercel]

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[tmeasday]

LGTM apart from one small mistake