Deliver icons as WEBP images for documentation

Currently, icons are displayed in the documentation as gray-scale images. Please deliver the new icons as follows:

• .webp format
• 16x16 size
• a color that is visible in both dark and light modes at doc.qt.io 

For the current style in the light and dark modes, see: https://doc-snapshots.qt.io/qtcreator-master/creator-how-to-show-and-hide-sidebars.html 

Changing the way the icons are show is also possible, within the limitations posed by the style sheet used for the HTML files at doc.qt.io and in the documentation packages in Qt Creator.