Doc: Add alternate text for Qt Data Visualization images

Alternate text (alt text) improves the documentation experience for
screen readers and for other accessibility tools. Also enable
reportmissingalttextforimages to enforce alt text on future images.

Task-number: QTBUG-135098
Pick-to: 6.11 6.12
Change-Id: I70ac0ddb7afa0ce4ddcdb669af9958caebcc0884
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>