Qt Positioning: Add alt texts

qdoc: 'QtPositioning': \image ../images/skyview_tableview.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtPositioning': \image ../images/rssiview_settings.webp is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtPositioning': \image ../images/example-weatherinfo.png is without a textual description, QDoc will not generate an alt text for the image.
qdoc: 'QtPositioning': \image permissions.png is without a textual description, QDoc will not generate an alt text for the image.