Add warning/info boxes to qdoc

qdoc alreay has support for note. However, that just leads to a bold "Note:" text. If we take a look at other documentation pages, e.g. MDN, they use colored boxes for highlighting specific  notes and warnings (e.g. on https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/input). 

Another example would be the blueish note boxes in the TypeScript documentation ( https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#the-primitives-string-number-and-boolean ).

We might not want to replace every occurrence of \note with such a box, but at least having the option would help giving the documentation a more modern look & feel and help highlighting specific sections.

Some care has to be taken to ensure that Creator is still able to properly render the documentation.