feat: optimize ux of the 2nd sticky nav on mobile

[sanyuan0704]

Summary

Optimize ui and interaction for nav in mobile device:

1. Make the toc list scrollable.
2. Optimize the interaction between the first nav and the second nav menu when user scrolls the page.
3. Add transition animation for the toc component enters when user click the On this page button.

preview:

nav-ux.mp4

Related Issue

#799

Checklist

• I have added changeset via pnpm run change.
• I have updated the documentation.
• I have added tests to cover my changes.

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