Storybook: Add redirect for moved introduction page

[mirka]

Follow-up to #76671

What?

Adds a URL redirect for the introduction docs page that was moved from Docs/Introduction to top-level Introduction in #76671.

Why?

The move changed the Storybook URL path from ?path=/docs/docs-introduction--page to ?path=/docs/introduction--page, which would break any existing bookmarks or shared links to the old URL.

How?

Added a redirect entry to the existing redirect script in storybook/manager-head.html. The pattern is anchored to the start of the path string to avoid false matches.

Testing Instructions

1. npm run storybook:dev
2. Visit the old URL directly: http://localhost:50241/?path=/docs/docs-introduction--page
3. Verify it redirects to http://localhost:50241/?path=/docs/introduction--page

Use of AI Tools

Cursor was used for initial drafting.

[ciampo]

🚀

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Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: ciampo <mciampini@git.wordpress.org>

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