feat: add active navigation highlighting example

[Akshu121796]

Summary

Adds a standalone Active Navigation Highlighting example under submissions/examples/active-navigation-highlighting.

The example demonstrates how documentation navigation items can automatically receive an active state based on the currently visible section using the Intersection Observer API.

Included

• Self-contained documentation-style demo
• Automatic active section detection on scroll
• Active navigation styling with visual indicators
• Responsive layout
• Documentation explaining usage and purpose

Checklist

• Added files only inside submissions/examples/active-navigation-highlighting
• Included demo.html
• Included style.css
• Included README.md
• Demo works without external frameworks
• Followed repository contribution guidelines

Closes #1320

[github-actions]

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Great job! All required files are present.

📁 Folder: submissions/examples/active-navigation-highlighting

Files found:

• ✅ demo.html
• ✅ style.css
• ✅ README.md

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• The maintainer will review your PR
• Respond to any feedback if requested
• Once approved, your contribution will be merged!

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[Akshu121796]

Hi @SAPTARSHI-coder,
I've submitted a PR for this issue.

Implemented:
• Active navigation highlighting based on the currently visible section
• Intersection Observer based section tracking
• Standalone demo showcasing documentation-style navigation
• README with usage and explanation

Thank you for reviewing.
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