docs: 📚 Update checkbox group and radio group docs regarding size variants

[MartaPintoTeixeira]

Description

Related ticket: consolidate sizes in input components (sd-radio/sd-checkbox/...)
#1326
After the closure of the ticket above it is now necessary to align Storybook docs with Figma docs.

Where to put the documentation

Storybook - checkbox group and radio group

DoR

• Item has business value
• Item has been estimated by the team
• Item is clear and well-defined
• Item dependencies have been identified

DoD

• Documentation has been created/updated (if applicable)
• Migration Guide has been created/updated (if applicable)
• Relevant stories (Features, A11y) are created/updated
• Implementation works successfully on feature branch