chore: improve keyboard/find behavior in doc editor

[stevenle]

Motivation

• Prevent the field deeplink anchor from being included in keyboard tab navigation so it doesn't interrupt tab order.
• Reduce spurious matches when using browser find (Ctrl+F) on the editor list by moving preview text out of the direct DOM text node.

Description

• Set the deeplink anchor to tabIndex={-1} in packages/root-cms/ui/components/DocEditor/DocEditor.tsx so the # link is skipped by tab navigation.
• Render the array item preview title using a data-preview attribute (data-preview={previewText}) instead of a direct text child in DocEditor.ArrayFieldPreview.
• Add a CSS rule in packages/root-cms/ui/components/DocEditor/DocEditor.css that displays the preview via .DocEditor__ArrayField__item__header__preview__title::before { content: attr(data-preview); } to preserve visual output while removing the preview from the element's text node.

Testing

• Ran git diff and git status to verify the changes were applied and staged, which succeeded.
• Committed the changes with message fix(cms): improve keyboard/find behavior in doc editor, which succeeded.
• Attempted an automated Playwright page screenshot to validate the UI, but the run failed due to no local web server responding (net::ERR_EMPTY_RESPONSE). No other automated tests were run per the request to avoid running tests unless explicitly prompted.

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Codex Task