Part of #1139 . Note: fixed in PMD 6.43.0 already.

This is especially useful for pages that are bigger than one screen. We have some space
on the right of the screen, where we can display a sticky toc box.

Note: for the mobile usage of the page, we probably have no space to have this toc always
visible. So for the start, we don't provide this here.

Possible library to use: https://afeld.github.io/bootstrap-toc/