feat(docs): Add scroll-to-top button on documentation pages

[adity1raut]

Summary

Long documentation pages require users to manually scroll back to the top,
which is a poor UX — especially on API reference pages with many DocSection entries.

Proposed Solution

Add a fixed "scroll to top" button that:

• Appears in the bottom-right corner after the user scrolls down 300px
• Smoothly scrolls back to the top on click
• Adapts to light and dark mode using existing CSS variables
• Is injected globally via Docusaurus's Root theme swizzle so it works on all doc pages

Implementation

• New component: src/components/ScrollToTopButton/
• Theme override: src/theme/Root.js (Docusaurus Root swizzle pattern)
• No changes needed to individual page or layout components