Storybook: Make "introduction" top level

[jasmussen]

What?

Followup/related to #76361.

Tiny PR to change the introduction.mdx file to not be inside a folder. Before:

After:

Why?

"Docs" has only introduction.mdx. If that's the only folder, it's just an extra click, and takes up another item of vertical space. The only reason to keep it would be to have additional files inside the docs folder, but conceptually they belong in named categories, e.g. foundations, components, etc.

Testing Instructions

npm run storybook:dev

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Co-authored-by: jasmussen <joen@git.wordpress.org>
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[im3dabasia]

LGTM ✅

[jasmussen]

Thank you for the review. The checks keep failing. This PR is tiny, and not urgent, but I'd appreciate a look from @WordPress/gutenberg-components

[mirka]

Failures are unrelated, I'll just go ahead and merge 👍

[ciampo]

Will this PR change the URL at which the introduction docs are available? Should we add a redirect from the old URL to the new URL?

[mirka]

Should we add a redirect from the old URL to the new URL?

Yeah sure, I can add that (#76701).