Add header to the right-hand table of content

[eriknson]

Current design for comparison

[alexandratran]

This doesn't seem possible with Docusaurus (at least without some custom plugin): https://docusaurus.io/docs/markdown-features/toc.

[ziad-saab]

it's possible. We have to create a customized component: https://docusaurus.io/docs/swizzling

[rekmarks]

I don't see why we should close this issue.