feat(legacy-html): add a smooth scrolling effect to the sidebar

[btea]

Description

I think adding a smooth scrolling effect would be fantastic. sync nodejs/node#59007

Validation

Related Issues

Check List

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run node --run test and all tests passed.
• I have check code formatting with node --run format & node --run lint.
• I've covered new added functionality with unit tests if necessary.

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Project	Deployment	Review	Updated (UTC)
api-docs-tooling	Ready	Preview	Dec 23, 2025 6:19am

[Copilot]

Pull request overview

This PR adds a smooth scrolling animation effect when the sidebar automatically scrolls to highlight the current page's navigation link on page load.

• Enhances user experience by adding smooth scrolling animation to sidebar navigation

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.82%. Comparing base (776dd0b) to head (84ebd02).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #528   +/-   ##
=======================================
  Coverage   79.82%   79.82%           
=======================================
  Files         121      121           
  Lines       12017    12017           
  Branches      840      840           
=======================================
  Hits         9593     9593           
  Misses       2421     2421           
  Partials        3        3           

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