[FEATURE] Intgrate CSS class "lead" for introductions

The most relevant text should always be highly visible.
Integrate the CSS class "lead" for introductions.

Lead brings a slightly larger font size and should be used
for the first short introduction text only.

"Lead" is a semantical annotation.
Follow-ups: Bring lead texts for all backend modules.

Change-Id: I10305c2d2ed66526d7e882aceff17b5d317cfbb3
Resolves: #51022
Releases: 6.2
Reviewed-on: https://review.typo3.org/23050
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters