Implement skeleton prop injection through OnyxPageLayout and OnyxForm

[MajaZarkova]

Why?

From @jannick-ux:

Skeleton modes are used for the initial loading of the entire page. So when I am directed to a page, the skeleton mode of the components is shown until the components themselves or the entire page is loaded.
Loading mode, on the other hand, is used when the components themselves no longer need to be loaded, but the data is processed or validated via a button click, for example (classic example “processing”).

To make this easier for the user and reflect this global/page state, we should enable the user to trigger the skeleton mode via an injection. the injection is already implemented. OnyxPageLyout and OnyxForm are providing this property. The issue arises when we have OnyxForm nested inside an OnyxPageLayout. For example if we set the skeleton prop to true on OnyxPageLayout layer, it doesn't reach the components inside the OnyxForm.

Acceptance criteria

• components inside an OnyxForm can inject (have access to) the skeleton prop on OnyxPageLayout layer, only if it's not set on OnyxForm layer.
• skeleton prop on OnyxForm layer tакеs advantage before the skeleton prop on OnyxPageLayout layer.
• A description for the prop is added and shown in stroybook

Implementation details

see

• https://github.com/SchwarzIT/onyx/blob/ebd8a1bda95b63920eabfd34250c00f0dc93a217/packages/sit-onyx/src/components/OnyxForm/OnyxForm.core.ts
• createCompute

Definition of Done

• covered by component tests
• screenshot tests are updated
• 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

Approval

• Storybook
• Deployed Docs
• approved by dev