Return the old path when next is missing from the url

[jordojordo]

Summary

Fixes #12542

This PR resolves the issue with old links (e.g., /extensions/extensions-getting-started) not properly redirecting to the latest version (/extensions/next/extensions-getting-started) for versioned extensions documentation.

Occurred changes and/or fixed issues

• Fixed the createRedirects function to handle unversioned routes (/extensions/...) and redirect them to the latest version (/extensions/next/...).
• Scoped the redirect logic to only affect unversioned routes (/extensions/...) and not interfere with existing versioned routes such as /extensions/v2/....

Technical notes summary

The createRedirects logic checks for routes under /extensions/next/ and generates redirects for their unversioned counterparts (/extensions/...). This ensures that unversioned links point to the latest version (v3) without impacting v2 or other explicitly versioned routes.

Areas or cases that should be tested

• Test unversioned links (e.g. /extensions/extensions-getting-started) and confirm they redirect to /extensions/next/extensions-getting-started.
• Test versioned links (/extensions/v2/extensions-getting-started) and confirm they load directly without being redirected.

Areas which could experience regressions

• Existing routes under /extensions/v2/... should be verified to ensure they are not unintentionally redirected.

Checklist

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• The PR has a Milestone
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• The PR has been self reviewed
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• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes

[aalves08]

LGTM