Highlight skipped levels in the table of content

Currently, the skipped level are either visible (< 15.2-rc-1)

or hidden (>= 15.2-rc-1)

But, skipped headings levels are an bade accessibility practice.

Based on the forum discussions (https://forum.xwiki.org/t/annotating-table-of-content-entries-skipped-level/12070), it seems interesting to make it possible to highlight skipped levels to the authors to encourage them to improve accessibility.

This could be done by making the skipped levels headings:

1. visible
2. with a explanation text on hover

Note that further discussion are required to decide if:

• when this must be activated (e.g., globally, based on some configuration...)
• other places where it would be interesting to present accessibility information to the writers (e.g., skipped level could be annotated with a information telling the author that a level is missing)