Docs: Cleanup with Promise instead of setTimeout

[JReinhold]

What I did

This PR changes the cleanup of stories in docs mode to be executed with a Promise, instead of setTimeout as introduced in #21214

When navigating between two docs entries, the setTimeout would cause teardown of the first set of stories to be called after renderToCanvas of the second set of stories. This results in the primary story from the second set of stories getting teared down and you end up with a blank canvas.

Using Promise.resolve instead makes sure that teardown happen before renderToCanvas, while still eliminating the React warning.

How to test

Navigate between two docs entries and see that there is no blank canvas anymore.

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[tmeasday]

👏