Validation example with radio/checkboxes

[minkyngkm]

As mentioned on MM, validation examples for checkboxes and radio buttons don't exist in the documentation https://vanillaframework.io/docs/base/forms#validation

It would be great if validation is available for checkboxes/radio buttons :)

[bartaz]

Scope of the issue:

• add an example of form validation with checkboxes (either as a new example or added to existing validation example)

Something like: https://codepen.io/bartoszopka/pen/abaKXJK