feat(client): add nested sections support with auto-expansion in sidebar

[Venkat-Entropik]

Description

This PR updates the sidebar to support nested sections (sub-sections inside groups).

Now the sidebar:

• Supports multiple levels (not just a flat list)
• Highlights the current page correctly

This makes navigation clearer, especially for deeply structured docs.

---

Validation

• Checked that parent sections expand when visiting pages inside them
• Checked that nested sections also expand correctly
• Verified it works for multiple levels
• Confirmed current page is highlighted properly

---

Related Issues

closes #8804

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Check List

• I have read the Contributing Guidelines and followed commit message rules.
• I have run pnpm format.
• I have run pnpm test.
• I have run pnpm build.
• Added tests if needed.

[cursor]

PR Summary

Medium Risk
Medium risk because it changes shared sidebar navigation rendering and selection logic, adding recursive nested items and auto-expansion which could affect active state and mobile select behavior across the site.

Overview
Adds nested (multi-level) navigation support to the sidebar, rendering sub-sections recursively and auto-expanding parent sections when the current pathname is inside them.

Updates WithSidebar to map site navigation into a nested items shape, and updates the mobile select to flatten nested entries so deep pages remain selectable. Includes new SidebarGroup styles and a Storybook story (NestedGroup) to demonstrate the nested behavior.

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Project	Deployment	Actions	Updated (UTC)
nodejs-org	Ready	Preview	Apr 7, 2026 4:35pm

[github-actions]

👋 Codeowner Review Request

The following codeowners have been identified for the changed files:

Team reviewers: @nodejs/nodejs-website

Please review the changes when you have a chance. Thank you! 🙏

[avivkeller]

Can you update the stories?

[github-actions]

Lighthouse Results

URL	Performance	Accessibility	Best Practices	SEO	Report
/en	🟢 99	🟢 93	🟢 96	🟢 100	🔗
/en/about	🟢 100	🟢 94	🟢 96	🟠 88	🔗
/en/about/previous-releases	🟢 97	🟢 97	🟢 100	🟢 100	🔗
/en/download	🟢 97	🟢 96	🟠 81	🟢 100	🔗
/en/download/archive/current	🟢 100	🟢 97	🟢 100	🟢 100	🔗
/en/blog	🟢 100	🟢 97	🟢 96	🟢 100	🔗

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.25%. Comparing base (9345c76) to head (a632529).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8806      +/-   ##
==========================================
+ Coverage   74.22%   74.25%   +0.02%     
==========================================
  Files         105      105              
  Lines        8870     8870              
  Branches      328      328              
==========================================
+ Hits         6584     6586       +2     
+ Misses       2284     2282       -2     
  Partials        2        2              

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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