Doc: Add alternate text for Qt Charts 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: Id2c189bdda91076d3391b7ebc017f59c6b9baab4
Reviewed-by: Kai Köhne <kai.koehne@qt.io>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
commitd11211b73d686a65105c2c9a7950e3ffb8065bd0
parent04e307da86b9fe98db8c3c2fe28ae4efd663407c
authorJerome Pasion <jerome.pasion@qt.io>
committerKai Köhne <kai.koehne@qt.io>
commit timeWed, 24 Jun 2026 07:13:01 +0000