[FEATURE] Spinner fine line
[FEATURE] Soft CSS3 transitions for sidebar
Cherry-Pick: [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