Improve CheckboxGroup doc

[KremnevDmitry]

Existing page: https://docs.jmix.io/jmix/flow-ui/vc/components/checkboxGroup.html

1. General:
   • Add a screenshot for orientations
   • Add examples of itemLabelGenerator, itemEnabledProvider and renderer
   • Add example of itemsEnum usage both XML and Java
   • Simplify code snippet
   • Add Style variants description and examples: See Vaadin doc.
2. Add attributes
   • ariaLabelledBy
   • clickShortcut
   • css
   • focusShortcut
3. Remove attributes:
   • requiredIndicatorVisible
   • invalid