Highlight current "on this page" title based on scroll position

[IgnaceMaes]

A common feature in documentation tools is highlighting the current position on page in the "on this page" section. This gives a clear indication to the user where they are currently at.

Examples:

• vitepress.dev/guide/markdown#internal-links
• docusaurus.io/docs/markdown-features/toc#example-section-1

Blocked by:

• Make TOC and "on this page" sidebars sticky #182