Correct how the progression is displayed in the table of content

[Jenselme]

We currently only display the progression if the title is visible. As soon as we scroll pass it, progression is lost. So we can be reading a section and not display anything in the progression bar. It’s the case in level 1 header as well as below headers.

It’s related to this bug in bootstrap. It’s currently flagged for 5.3.4, so let’s see how it evolves before doing anything.

Initial implementation done in #157