feat(site): add scroll-to-top button for improved navigation

[Mayankaggarwal8055]

Summary

This PR adds a scroll-to-top button to improve navigation on long documentation and example pages.

Details

• Displays button after user scrolls down
• Enables smooth scrolling back to top
• Integrated across layout templates (basic, doc, example)

Motivation

Documentation pages can become lengthy, requiring excessive manual scrolling.
This enhancement improves usability and user experience without affecting existing layout structure.

Testing

Tested locally across affected layouts to ensure:

• Button visibility triggers correctly on scroll
• Smooth scrolling behavior works as expected
• No layout breakage introduced

Fixes: apache/echarts#21516

[Mayankaggarwal8055]

@apache/echarts-website-maintainers

Fixes apache/echarts#21516 (UX enhancement for echarts.apache.org)

Ready for review! ✅

• 3 layouts updated (basic/example/doc.html)
• Mobile responsive Scroll-To-Top button
• Production-ready + fully tested