Accessibility docs: define guiding rules for grouping, labels and disabled state

[gfellerph]

The accessibility docs at https://design-system.post.ch/?path=/docs/46da78e8-e83b-4ca1-aaf6-bbc662efef14--docs are incomplete.

### Tasks
- [x] **Labels**: Write accessibility guide about form labels (visible and invisible), aria-label, ...
- [ ] https://github.com/swisspost/design-system/issues/2261