Style "on this page" to use header depth info

[IgnaceMaes]

The "on this page" section currently doesn't take into account which level the header is. We could use this information to make the section more clear:

• Should all headers be included? Should we e.g. omit headers on level 4 and below?
• Should we indent headers to represent their respective level?
  • VitePress only shows h2
    • e.g. https://vitepress.dev/guide/markdown#internal-links
  • DocuSaurus uses indents and only shows h2/h3
    • e.g. https://docusaurus.io/docs/markdown-features/toc#example-section-1

Related to #174