Improve DropdownButton doc

[KremnevDmitry]

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

1. General
   • New clickHandlers should be described with generated handlers code snippets
   • Update code snippet for actionItem -> notifications.create("Phone number: +6(876)5463").show() to notifications.show("Phone number: +6(876)5463"). The same for componentItem
2. Add attributes
   • css
   • focusShortcut
   • overlayClass