[documentation] Icons class names have to be updated

[OfficierAzarov]

Current behaviour: examples given on this page don't work.

Fix proposal: update class names according to the new DSFR naming conventions by replacing "ri" prefix with the "fr-fi" prefix.

import Icon from './Icon';
import Link from '../../interface/Link';

<Link
    title="title"
    icon="ri-twitter-line"
    iconPosition="left"
    >this is a link
</Link>

would become

import Icon from './Icon';
import Link from '../../interface/Link';

<Link
    title="title"
    icon="fr-fi-twitter-line"
    iconPosition="left"
    >this is a link
</Link>

[poplingue]

Documentation is broken because of styleguidist