Improve DatePicker doc

[KremnevDmitry]

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

1. General:
   • Date range description using XML markup. Static dates can be set with yyyy-MM-dd format using min and max element's attributes.
   • Add Style variants description and examples: See Vaadin doc
   • Add example for data binding
2. Add attributes:
   • css
   • focusShortcut
   • ariaLabel, ariaLabelledBy
   • overlayClass
3. Remove attributes:
   • invalid
   • requiredIndicatorVisible
4. Add elements:
   • Add custom validator
   • prefix