Document data-active attribute for active state CSS targeting

[mintlify]

Summary

• Added an "Active state targeting" section to the custom scripts page documenting the new data-active attribute on TOC items, sidebar groups, and sidebar nav items
• Includes CSS examples for styling active/inactive states

Related: mintlify/mint#6550

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Note

Low Risk
Documentation-only change that adds guidance for styling active navigation/TOC elements; no runtime code or behavior changes.

Overview
Updates customize/custom-scripts.mdx to document active-state CSS targeting by noting a data-active attribute on SidebarGroup, sidebar nav items, and TableOfContentsItem, including example selectors like sidebar-group[data-active] and toc-item[data-active].

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