Core: Pass parameters in SET_INDEX for docs entries

[tmeasday]

Closes #21798 for SSv6

What I did

Include parameters in the (v6) SET_STORIES event.

How to test

See E2E spec.

In a ssv6 sandbox, visit:

• http://localhost:6006/?path=/docs/addons-docs-stories-mdx-unattached--docs
• http://localhost:6006/?path=/docs/example-button--docs

And check the backgrounds toolbar item.

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"]

[yannbf]

LGTM, apart from the fact that the addon-backgrounds toolbar makes no sense (currently, at least) in unattached docs when there are no stories.

Also, could you check the Chromatic diffs?