Footer: small variant

[gfellerph]

There is a small variant of the footer available.

Design: https://www.figma.com/file/xZ0IW0MJO0vnFicmrHiKaY/Components-Post?type=design&node-id=185%3A260&mode=design&t=mGeQ7btbwNVae33N-1

### Tasks
- [ ] https://github.com/swisspost/design-system/issues/1586
- [ ] Implement the small variant of the footer as an option on `<swisspost-internet-footer>`
- [ ] Document in storybook
- [ ] Design review
- [ ] Test in high contrast mode