Storybook: Keep Docs before named attached MDX pages - #85308

What?

Keep the Docs page above named attached MDX pages, including TextareaControl's Migration Guide, in the Storybook sidebar.

Follow-up to #84563.

Why?

The existing Docs-first comparison works for entries sharing a title, but the earlier hierarchy comparison treats a named attached MDX page as a standalone page. For example, TextareaControl's Migration Guide is prioritized ahead of neighboring components while its Docs page is sorted alphabetically. These contradictory comparisons allow the migration guide to appear above Docs in the complete index.

How?

Exclude named attached MDX entries from the hierarchy's standalone-MDX priority. Entries named Docs retain that priority, preserving nested documentation pages such as Badge's Usage Guidelines. The existing same-title priority then puts Docs before the migration guide and examples.

This also fixes the same ordering issue in CheckboxControl, InputControl, RadioControl, ToggleControl, Tabs, Menu, and Combobox.

Extend the existing sidebar validator to require every Docs page to precede all other entries in its folder, including nested documentation. Run it against the complete generated index in the existing Storybook CI job.

Testing Instructions

1. Open TextareaControl's Docs page in Storybook, using ?path=/docs/components-textareacontrol--docs.
2. Confirm its sidebar entries appear in this order: Docs, Migration Guide, Default.
3. Open CheckboxControl, InputControl, RadioControl, and ToggleControl, and confirm Docs appears before Migration Guide and the examples.
4. Check Tabs, Menu, and Combobox. Confirm Docs appears before Best Practices or Usage guidelines, followed by the examples.
5. Check Badge and confirm Docs still appears before the nested Usage Guidelines page. Check Form and confirm Best Practices and Checkbox Groups remain above its component folders.

Screenshots or screencast

Before	After