Improve Icon docs

[Flaurite]

Description

1. Add attributes:
   • css
   • clickShortcut

Vaadin provides fontIcon and svgIcon components (see Vaadin docs). Jmix does not extent them but allows users to use them in XML. I think we can mention about this components (e.g. simple example with XML) or at least provide a link to Vaadin docs.

[iudaev]

Describing several components in one article may be confusing. It would be better to create separate articles for fontIcon and svgIcon, or describe working with icons in a separate section, as it was in the classic documentation https://docs.jmix.io/1.x/jmix/1.5/ui/icons.html