Preview API

TODO write proper documentation of this package

"Sub packages" README documents

This package used to be multiple packages (they have been combined into this one):

@storybook/addons read (old) docs
@storybook/client-api read (old) docs
@storybook/core-client read (old) docs
@storybook/preview-web read (old) docs
@storybook/store read (old) docs