docs: honor PREVIEW_URL in non-inlined stories

[ittaibaratz]

Is your feature request related to a problem? Please describe

For our internal build system, we need PREVIEW_URL to be honored not just for normal stories, but also for non-inlined stories, as we use the docs add-on in some of our newer stories.

Describe the solution you'd like

non-Inline stories in docs (inlineStories = false) should respect PREVIEW_URL as they iframe the stories.

Describe alternatives you've considered

The only alternative is to inline our stories, but this is impossible because of their styles.

Are you able to assist to bring the feature to reality?
Yes! I've made this simple PR to enable this feature: #16773 , if someone can review it we can continue the conversation there. I've tested this PR locally against my project and it works well.

Additional context
Add any other context or screenshots about the feature request here.

[shilman]

Jiminy cricket!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.5.0-alpha.3 containing PR #16773 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease

Closing this issue. Please re-open if you think there's still more to do.