Implement OnyxNavButton and OnyxFlyoutMenu disabled state

[MajaZarkova]

Why?

We need a disabled state for the OnyxNavButton

Design

Figma

Acceptance criteria

• 'cursor: not-allowed' on hover when it's disabled
• disabled state meets the style in Figma

Definition of Done

• The following component requirements are implemented:
  • example usage added to apps/demo-app/src/views/HomeView.vue
• covered by tests
  • functional tests (Playwright or unit test)
  • visual tests (Playwright screenshots)
• follow-up tickets were created if necessary
• updated version + documentation is deployed
• Storybook can show the feature
• Storybook code snippet of new/changed examples are checked that they are generated correctly
• Namings are aligned with Figma

Approval

• Storybook
• Deployed Docs
• approved by designer @jannick-ux
• approved by dev