Add documentation for validators, file utils and i18n

[astagi]

Description

Add documentation for:

• ItValidators
• FileUtils
• i18n

Checklist

• Verify and update, markup and classes of the resulting DOM template, compared to Bootstrap Italia 2
• Check the rendering of the component (CSS application including spacing, dimensions, typography, ...), compared to Bootstrap Italia and the new UI kit
• Check the behavior of the component (JavaScript, user interaction, states, keyboard interaction for accessibility, ...), compared to Bootstrap Italia 2
• Verify the accessibility of the component, including automatic tests and manual evaluations by a11y experts, if possible
• Evaluate the need to supplement documentation with more detailed information.
• Write tests for this component
• Write documentation for this component

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. Please read #127.