Improve Button doc

[KremnevDmitry]

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

1. General:

   • Add Live Demo link for the Variants section: https://demo.jmix.io/ui-samples/sample/badge-theme-variant

   • The XML tags in the code snippet is not closed for the Action section

   • Lumo icon can be set not only programmatically, but also in XML <button icon="lumo:menu"/>

   • New clickHandlers should be described with generated handlers screenshots/code snippets (cause the subject attribute is new)

   • Add a tip for overriding properties from bound action See io.jmix.flowui.kit.component.button.JmixButtonActionSupport#setAction The following attributes will be overrided by default:

2. Add attributes

   • ariaLabel, ariaLabelledBy
   • css
   • focusShortcut
   • tabIndex

3. Add elements [seems useless, to be discussed]

   • suffix
   • prefix

[glebfox]

Also, describe how to add click shortcut. Take a look at this issue and comment: https://github.com/jmix-framework/jmix/issues/2645#issuecomment-1929421608