Make all icons used in docs gray with transparent background

Currently, black icons with a white background stick out in the docs, while black icons with a transparent background become invisible. In the Qt Design Studio Manual, we also have gray icons with a transparent background, which work nicely.

Because it would be difficult to make CSS changes that take all these cases into account, we should pick an icon style that looks good in both light and dark modes and then fix any icons that don't adhere to the style in the Qt Creator Manual and Qt Design Studio Manual.

This page shows black icons with transparent and white backgrounds as well as gray-scale icons with a white background.

• https://doc.qt.io/qtdesignstudio/qtquick-positioning.html#summary-of-aligment-buttons

Here, the icons look bad in the dark mode:

• https://doc.qt.io/qtcreator/creator-sidebars.html

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The icons have now all been moved to qtcreator\doc\qtcreator\images\icons and can be batch-edited.