In `library`, add component `skeleton`

Description

As a user of the Ouds library
I want to be able to use the component
So that I can add this Ouds component in my own projects

As a user of the library
I want to be able to use the component
So that I have access to the documentation on how to use it

As a user of the Demo App
I want to access the Component
So that I can understand how to use it:

• while using the demo app I can interact with the available options and observe the changes on the component itself
• I can see the code associated to the component customization
• I can copy and paste the code from the app based on the selected customization
• or when looking into the code of the app itself, I would also understand how to use it

Specifications

See Figma Skeleton specifications

Definition of Ready

• Need to update Figma with these specifications coming from dev/design meeting (10.09.2026):
  - Shimmer effect must be applied on whole screen from left to right on mobile.
  - All shapes must be allowed

Definition of Done

• Add component skeleton (API, doc, demo app) with shimmer effect and circle shape
• Add component skeleton (API, doc, demo app) with shimmer effect and capsule shape
• Add component skeleton (API, doc, demo app) with shimmer effect and rectangle shape with square corners
• Add component skeleton (API, doc, demo app) with shimmer effect and rectangle shape with rounded corners
• Add component skeleton (API, doc, demo app) with shimmer effect and square shape
• Add snapshots tests for skeleton

Start with the Figma Skeleton specifications and the existing library component and Demo App patterns. Implement the skeleton API, documentation, demo customizations, shimmer variants and listed shapes, then add snapshot tests. Done means all five shape variants are available across the API, documentation and demo app with snapshot coverage.